Investments - Schedule of Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 630,447	$ 638,333
Gross Unrealized Gains	1,909	32
Gross Unrealized Losses	0	1,487
Fair Value	632,356	636,878
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	256,685
Gross Unrealized Gains	154
Gross Unrealized Losses	0
Fair Value	256,839
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	259,538	509,783
Gross Unrealized Gains	84	32
Gross Unrealized Losses	0	0
Fair Value	259,622	509,815
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	102,366	101,587
Gross Unrealized Gains	1,547	0
Gross Unrealized Losses	0	1,482
Fair Value	103,913	100,105
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,858	26,963
Gross Unrealized Gains	124	0
Gross Unrealized Losses	0	5
Fair Value	$ 11,982	$ 26,958