| **Selected Quarterly Financial Data (Details) (USD $)In Thousands, except Per Share data, unless otherwise specified** | 3 Months Ended                                           | 12 Months Ended                                          |
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| **Quarterly Financial Data [Abstract]**                                                                                |                                                     |                                                     |   |   |   |   |   |   |   |   |   |
| Operating Revenues                                                                                                     | $ 185,821us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 164,187us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 462,247us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 657,733us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 200,000us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 162,943us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 399,411us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 515,875us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 1,469,988us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 1,278,229us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 1,122,780us-gaap_RegulatedAndUnregulatedOperatingRevenue |
| Margin                                                                                                                 | 112,326us-gaap_GrossProfit                               | 104,847us-gaap_GrossProfit                               | 211,523us-gaap_GrossProfit | 261,512us-gaap_GrossProfit | 109,011us-gaap_GrossProfit | 97,000us-gaap_GrossProfit | 183,856us-gaap_GrossProfit | 231,623us-gaap_GrossProfit | 690,208us-gaap_GrossProfit | 621,490us-gaap_GrossProfit | 575,446us-gaap_GrossProfit |
| Operating Income                                                                                                       | 6,993us-gaap_OperatingIncomeLoss                         | 3,254us-gaap_OperatingIncomeLoss                         | 67,299us-gaap_OperatingIncomeLoss | 102,319us-gaap_OperatingIncomeLoss | 5,886us-gaap_OperatingIncomeLoss | 591us-gaap_OperatingIncomeLoss | 51,504us-gaap_OperatingIncomeLoss | 86,213us-gaap_OperatingIncomeLoss | 179,865us-gaap_OperatingIncomeLoss | 144,194us-gaap_OperatingIncomeLoss | 125,723us-gaap_OperatingIncomeLoss |
| Net income (loss)                                                                                                      | $ (8,967)us-gaap_NetIncomeLoss                           | $ (7,344)us-gaap_NetIncomeLoss                           | $ 62,540us-gaap_NetIncomeLoss | $ 97,572us-gaap_NetIncomeLoss | $ (5,003)us-gaap_NetIncomeLoss | $ (2,293)us-gaap_NetIncomeLoss | $ 55,790us-gaap_NetIncomeLoss | $ 85,923us-gaap_NetIncomeLoss | $ 143,801us-gaap_NetIncomeLoss | $ 134,417us-gaap_NetIncomeLoss | $ 119,847us-gaap_NetIncomeLoss |
| **Earnings Per Share of Common Stock:**                                                                                |                                                     |                                                     |   |   |   |   |   |   |   |   |   |
| Basic (usd per share)                                                                                                  | $ (0.11)us-gaap_EarningsPerShareBasic                    | $ (0.09)us-gaap_EarningsPerShareBasic                    | $ 0.80us-gaap_EarningsPerShareBasic | $ 1.27us-gaap_EarningsPerShareBasic | $ (0.07)us-gaap_EarningsPerShareBasic | $ (0.03)us-gaap_EarningsPerShareBasic | $ 0.74us-gaap_EarningsPerShareBasic | $ 1.19us-gaap_EarningsPerShareBasic | $ 1.85us-gaap_EarningsPerShareBasic | $ 1.80us-gaap_EarningsPerShareBasic | $ 1.67us-gaap_EarningsPerShareBasic |
| Diluted (usd per share)                                                                                                | $ (0.11)us-gaap_EarningsPerShareDiluted                  | $ (0.09)us-gaap_EarningsPerShareDiluted                  | $ 0.80us-gaap_EarningsPerShareDiluted | $ 1.26us-gaap_EarningsPerShareDiluted | $ (0.07)us-gaap_EarningsPerShareDiluted | $ (0.03)us-gaap_EarningsPerShareDiluted | $ 0.74us-gaap_EarningsPerShareDiluted | $ 1.18us-gaap_EarningsPerShareDiluted | $ 1.84us-gaap_EarningsPerShareDiluted | $ 1.78us-gaap_EarningsPerShareDiluted | $ 1.66us-gaap_EarningsPerShareDiluted |

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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.
+ References
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| - Definition
Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.
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| X                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The net result for the period of deducting operating expenses from operating revenues.
+ References
No definition available.
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| - Definition
The total amount of operating revenues recognized during the period.
+ References
No definition available.
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