Investments - Schedule of Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 350,763	$ 638,333
Gross Unrealized Gains	246	32
Gross Unrealized Losses	0	1,487
Fair Value	351,009	636,878
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	98,962
Gross Unrealized Gains	22
Gross Unrealized Losses	0
Fair Value	98,984
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	234,674	509,783
Gross Unrealized Gains	187	32
Gross Unrealized Losses	0	0
Fair Value	234,861	509,815
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,269	101,587
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	1,482
Fair Value	5,269	100,105
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,858	26,963
Gross Unrealized Gains	37	0
Gross Unrealized Losses	0	5
Fair Value	$ 11,895	$ 26,958