Investments - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 40,956	$ 191,240
Less Than 12 Months, Unrealized Losses	1,388	1,117
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	0	184,740
Less Than 12 Months, Unrealized Losses	0	79
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	18,262	6,292
Less Than 12 Months, Unrealized Losses	1,152	981
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	22,694	208
Less Than 12 Months, Unrealized Losses	236	57
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	$ 0	$ 0