| **Long-Term Debt Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands** |                |               |           |               |                 |                | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                      |        |
|---------------------------------------------------------------------------------------------------|----------------|---------------|-----------|---------------|-----------------|----------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------|
||
| **Debt Instrument**                                                                               |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Cash repayment of convertible notes                                                               |           |          |      |          |            |           | $ 115,000                                                                                                                                                                                                                                                                                                                                                                           | $ 0    |   |
| 3.25% Convertible Senior Notes due 2016                                                           |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| **Debt Instrument**                                                                               |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Debt Instrument, Interest Rate, Stated Percentage                                                 |           |          |      |          |            |           | 3.25%                                                                                                                                                                                                                                                                                                                                                                               |   |   |
| 3.25% Convertible Note, Conversion Price                                                          |           |          |      |          |            | $ 42.40        |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Debt Instrument, Issuance Date                                                                    |           |          |      |          |            | Nov. 15,  2010 |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Debt Instrument, Maturity Date                                                                    |           |          |      | May 15,  2016 |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| 7.75% Senior Notes due 2022                                                                       |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| **Debt Instrument**                                                                               |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Debt Instrument, Interest Rate, Stated Percentage                                                 |           |          |      |          |            |           | 7.75%                                                                                                                                                                                                                                                                                                                                                                               |   |   |
| Debt Instrument, Issuance Date                                                                    |           |          |      |          | Oct.  03,  2012 |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Debt Instrument, Maturity Date                                                                    | Oct. 15,  2022 |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Revolving Credit Facility                                                                         |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| **Debt Instrument**                                                                               |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Line of Credit Facility, Maximum Borrowing Capacity ($)                                           |           |          |      |          |            |           | $ 1,000,000                                                                                                                                                                                                                                                                                                                                                                         |   |   |
| Line of Credit Facility, Current Borrowing Capacity ($)                                           |           |          |      |          |            |           | 450,000                                                                                                                                                                                                                                                                                                                                                                             |   |   |
| Line of Credit Facility, Increase (Decrease), Net                                                 |           |          |      |          |            |           | $ 250,000                                                                                                                                                                                                                                                                                                                                                                           |   |   |
| Line of Credit Facility, Expiration Date                                                          |           | May 21,  2020 |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Debt Instrument, Covenant Description                                                             |           |          |      |          |            |           | The revolving credit facility contains covenants customary for agreements of this type, with the most restrictive being certain financial tests on a quarterly basis. The financial tests, as defined per the revolving credit facility, include requirements to: (a) maintain a minimum current ratio of 1.00 to 1.00 and (b) not exceed a maximum leverage ratio of 4.25 to 1.00. |   |   |
| Line of Credit Facility, Remaining Borrowing Capacity                                             |           |          |      |          |            |           | $ 438,300                                                                                                                                                                                                                                                                                                                                                                           |   |   |
| RNG Credit Facility [Member]                                                                      |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| **Debt Instrument**                                                                               |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| PDC Irrevocable Standby Letter of Credit ($)                                                      |           |          |      |          |            |           | $ 11,700                                                                                                                                                                                                                                                                                                                                                                            |   |   |
| First Payment | 7.75% Senior Notes due 2022                                                       |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| **Debt Instrument**                                                                               |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Debt Instrument, Frequency of Periodic Payment                                                    |           |          |      |          |            |           | April 15                                                                                                                                                                                                                                                                                                                                                                            |   |   |
| Second Payment | 7.75% Senior Notes due 2022                                                      |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| **Debt Instrument**                                                                               |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Debt Instrument, Frequency of Periodic Payment                                                    |           |          |      |          |            |           | October 15                                                                                                                                                                                                                                                                                                                                                                          |   |   |
| Revolving Credit Facility                                                                         |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| **Debt Instrument**                                                                               |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Long-term Line of Credit                                                                          |           |          |      |          |            |           | $ 0                                                                                                                                                                                                                                                                                                                                                                                 |   | $ 37,000 |
| Weighted Average Interest Rate                                                                    |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   | 2.60% |
| Maximum Borrowing Base [Member] | Revolving Credit Facility                                       |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| **Debt Instrument**                                                                               |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Line of Credit Facility, Current Borrowing Capacity ($)                                           |           |          |      |          |            |           | $ 700,000                                                                                                                                                                                                                                                                                                                                                                           |   |   |
| 3.25% Convertible Senior Notes due 2016                                                           |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| **Debt Instrument**                                                                               |           |          |      |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Cash repayment of convertible notes                                                               |           |          | $ 115,000 |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| Shares issued upon maturity of convertible notes                                                  |           |          | 792,406   |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |
| 3.25% Convertible Note, Conversion Price                                                          |           |          | $ 42.40   |          |            |           |                                                                                                                                                                                                                                                                                                                                                                                |   |   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The number of shares issued in exchange for the original debt being converted in a noncash (or part noncash) transaction. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or payments in the period.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The price per share of the conversion feature embedded in the debt instrument.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Description of minimum financial levels (for example, tangible net worth and working capital) and achievement of certain financial ratios (for example, working capital ratio and debt service coverage ratio), and adherence to certain clauses which generally require or restrict certain actions (for example, entering into a debt arrangement with equal or greater seniority, and selling or discontinuing a certain business segment or material subsidiary) to be in compliance with the covenant clauses of the debt agreement. May also include a discussion of the adverse consequences that would result if the entity violates or fails to satisfy the covenants.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Description of the frequency of periodic payments (monthly, quarterly, annual).
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Contractual interest rate for funds borrowed, under the debt agreement.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Date the debt instrument was issued, in CCYY-MM-DD format.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
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No definition available.
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| - Definition
Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.
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| X                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Weighted average interest rate of debt outstanding.
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No definition available.
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| X                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.
+ References
No definition available.
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Date the credit facility terminates, in CCYY-MM-DD format.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of increase (decrease) of the credit facility.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The cash outflow from the repayment of a long-term debt instrument which can be exchanged for a specified amount of another security, typically the entity's common stock, at the option of the issuer or the holder.
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