Investments - Investments Classified as Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 199,429	$ 646,273
Gross Unrealized Gains	0	32
Gross Unrealized Losses	1,117	1,487
Fair Value	198,312	644,818
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	184,819	509,783
Gross Unrealized Gains	0	32
Gross Unrealized Losses	79	0
Fair Value	184,740	509,815
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,273	101,587
Gross Unrealized Gains	0	0
Gross Unrealized Losses	981	1,482
Fair Value	6,292	100,105
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	265	26,963
Gross Unrealized Gains	0	0
Gross Unrealized Losses	57	5
Fair Value	208	26,958
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	192,357	638,333
Gross Unrealized Gains	0	32
Gross Unrealized Losses	1,117	1,487
Fair Value	191,240	636,878
Guaranteed income fund
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,072	7,940
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 7,072	$ 7,940