Investments - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 9,901	$ 36,648
Less Than 12 Months, Unrealized Losses	25	2,050
12 Months or Greater, Fair Value	25,209	35,832
12 Months or Greater, Unrealized Losses	1,258	1,157
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	9,901	9,870
Less Than 12 Months, Unrealized Losses	25	22
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	0	15,515
Less Than 12 Months, Unrealized Losses	0	691
12 Months or Greater, Fair Value	21,982	29,595
12 Months or Greater, Unrealized Losses	1,245	1,126
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value		11,263
Less Than 12 Months, Unrealized Losses		1,337
12 Months or Greater, Fair Value		1,986
12 Months or Greater, Unrealized Losses		18
Short-term bond
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	0	0
Less Than 12 Months, Unrealized Losses	0	0
12 Months or Greater, Fair Value	3,227	4,251
12 Months or Greater, Unrealized Losses	$ 13	$ 13