Investments - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 36,648	$ 68,742
Less Than 12 Months, Unrealized Losses	2,050	1,416
12 Months or Greater, Fair Value	35,832	7,124
12 Months or Greater, Unrealized Losses	1,157	470
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	9,870	0
Less Than 12 Months, Unrealized Losses	22	0
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	15,515	64,516
Less Than 12 Months, Unrealized Losses	691	1,410
12 Months or Greater, Fair Value	29,595	6,971
12 Months or Greater, Unrealized Losses	1,126	359
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	11,263	0
Less Than 12 Months, Unrealized Losses	1,337	0
12 Months or Greater, Fair Value	1,986	153
12 Months or Greater, Unrealized Losses	18	111
Short-term bond
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	0	4,226
Less Than 12 Months, Unrealized Losses	0	6
12 Months or Greater, Fair Value	4,251	0
12 Months or Greater, Unrealized Losses	$ 13	$ 0