Investments - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 191,240	$ 219,512
Less Than 12 Months, Unrealized Losses	1,117	1,310
12 Months or Greater, Fair Value	0	14,260
12 Months or Greater, Unrealized Losses	0	188
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	184,740	109,984
Less Than 12 Months, Unrealized Losses	79	6
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	6,292	85,845
Less Than 12 Months, Unrealized Losses	981	1,294
12 Months or Greater, Fair Value	0	14,260
12 Months or Greater, Unrealized Losses	0	188
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	208	23,683
Less Than 12 Months, Unrealized Losses	57	10
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	$ 0	$ 0