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

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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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.

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