| **Investments - Schedule of Investments (Details) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                                          | Dec. 31, 2013                                                                                                                                                          |
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| **Schedule of Available-for-sale Securities [Line Items]**                                          |                                                                                                                                                                   |                                                                                                                                                                   |
| Amortized Cost                                                                                      | $ 638,333us-gaap_AvailableForSaleSecuritiesAmortizedCost                                                                                                               | $ 149,097us-gaap_AvailableForSaleSecuritiesAmortizedCost                                                                                                               |
| Gross Unrealized Gains                                                                              | 32us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax                                                                                            | 88us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax                                                                                            |
| Gross Unrealized Losses                                                                             | 1,487us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax                                                                                         | 2,213us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax                                                                                         |
| Fair Value                                                                                          | 636,878us-gaap_AvailableForSaleSecurities                                                                                                                              | 146,972us-gaap_AvailableForSaleSecurities                                                                                                                              |
| U.S. treasury securities                                                                            |                                                                                                                                                                   |                                                                                                                                                                   |
| **Schedule of Available-for-sale Securities [Line Items]**                                          |                                                                                                                                                                   |                                                                                                                                                                   |
| Amortized Cost                                                                                      | 509,783us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasurySecuritiesMember                            | 124,861us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasurySecuritiesMember                            |
| Gross Unrealized Gains                                                                              | 32us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasurySecuritiesMember       | 88us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasurySecuritiesMember       |
| Gross Unrealized Losses                                                                             | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasurySecuritiesMember        | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasurySecuritiesMember        |
| Fair Value                                                                                          | 509,815us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasurySecuritiesMember                                         | 124,949us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasurySecuritiesMember                                         |
| Corporate debt securities                                                                           |                                                                                                                                                                   |                                                                                                                                                                   |
| **Schedule of Available-for-sale Securities [Line Items]**                                          |                                                                                                                                                                   |                                                                                                                                                                   |
| Amortized Cost                                                                                      | 101,587us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                         | 24,236us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                          |
| Gross Unrealized Gains                                                                              | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember     | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember     |
| Gross Unrealized Losses                                                                             | 1,482us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember | 2,213us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember |
| Fair Value                                                                                          | 100,105us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                      | 22,023us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                       |
| Preferred stock                                                                                     |                                                                                                                                                                   |                                                                                                                                                                   |
| **Schedule of Available-for-sale Securities [Line Items]**                                          |                                                                                                                                                                   |                                                                                                                                                                   |
| Amortized Cost                                                                                      | 26,963us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                   |                                                                                                                                                                   |
| Gross Unrealized Gains                                                                              | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember              |                                                                                                                                                                   |
| Gross Unrealized Losses                                                                             | 5us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember              |                                                                                                                                                                   |
| Fair Value                                                                                          | $ 26,958us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                              |                                                                                                                                                                   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
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| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
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| - Definition
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
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| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ References
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