| **Investments (Tables)**                               | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|--------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Investments, Debt and Equity Securities [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
| Schedule of Investments                                | At September 30, 2015 investments classified as available-for-sale securities were as follows: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair ValueDebt securities:       Commercial paper (1)$98,962 $22 $— $98,984U.S. Treasury securities234,674 187 — 234,861Corporate debt securities5,269 — — 5,269Preferred stock11,858 37 — 11,895 $350,763 $246 $— $351,009    (1)Recorded in Cash and cash equivalents on the Company’s Condensed Consolidated Balance Sheets.    At December 31, 2014 investments classified as available-for-sale securities were as follows:  Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair ValueDebt securities:       U.S. Treasury securities$509,783 $32 $— $509,815Corporate debt securities101,587 — 1,482 100,105Preferred stock26,963 — 5 26,958 $638,333 $32 $1,487 $636,878 |
| Continuous Unrealized Loss Position                    | The Company has revised this disclosure in the following table that provides the corporate debt securities continuous unrealized loss position and its related fair values:     As of September 30, 2015 As of December 31, 2014 Less Than 12 Months 12 Months or Greater Less Than 12 Months 12 Months or Greater Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized LossesCorporate debt securities$— $— $— $— $85,845 $1,294 $14,260 $188                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
| Contractual Maturities of Investments                  | The net carrying value and estimated fair value of investments classified as available-for-sale at September 30, 2015, by contractual maturity are shown in the following table. Actual maturities may differ from contractual maturities because certain borrowers have the right to call or prepay obligations.  Amortized Cost Fair ValueDue in one year or less$333,636 $333,845Due after one year through five years5,269 5,269 338,905 339,114Preferred stock 11,858 11,895 $350,763 $351,009                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of fair value of investments in debt and equity securities in an unrealized loss position categorized neither as held-to-maturity nor trading securities.
+ References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValueTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62586-112803Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of maturities of an entity's investments as well as any other information pertinent to the investments.
+ References
No definition available.
+ Details**Name:**us-gaap_InvestmentsClassifiedByContractualMaturityDateTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |