Investments - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 68,742	$ 191,240
Less Than 12 Months, Unrealized Losses	1,416	1,117
12 Months or Greater, Fair Value	7,124	0
12 Months or Greater, Unrealized Losses	470	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	64,516	6,292
Less Than 12 Months, Unrealized Losses	1,410	981
12 Months or Greater, Fair Value	6,971	0
12 Months or Greater, Unrealized Losses	359	0
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	0	208
Less Than 12 Months, Unrealized Losses	0	57
12 Months or Greater, Fair Value	153	0
12 Months or Greater, Unrealized Losses	111	0
Short-term bond
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	4,226	0
Less Than 12 Months, Unrealized Losses	6	0
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	$ 0	$ 0