Investments - Schedule of Investments (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 633,884,000	$ 638,333,000
Gross Unrealized Gains	630,000	32,000
Gross Unrealized Losses	780,000	1,487,000
Fair Value	633,734,000us-gaap_AvailableForSaleSecurities	636,878,000us-gaap_AvailableForSaleSecurities
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	248,294,000
Gross Unrealized Gains	60,000
Gross Unrealized Losses	0
Fair Value	248,354,000
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	259,923,000	509,783,000
Gross Unrealized Gains	6,000	32,000
Gross Unrealized Losses	0	0
Fair Value	259,929,000	509,815,000
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	101,973,000	101,587,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	780,000	1,482,000
Fair Value	101,193,000	100,105,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	49,100,000	14,300,000
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,694,000	26,963,000
Gross Unrealized Gains	564,000	0
Gross Unrealized Losses	0	5,000
Fair Value	$ 24,258,000	$ 26,958,000

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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