Investments - Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investments
Less Than 12 Months, Fair Value	$ 9,946
Less Than 12 Months, Unrealized Losses	15
12 Months or Greater, Fair Value	7,398
12 Months or Greater, Unrealized Losses	827
Less Than 12 Months, Fair Value		$ 36,648
Less Than 12 Months, Unrealized Losses		2,050
12 Months or Greater, Fair Value		35,832
12 Months or Greater, Unrealized Losses		1,157
Unrestricted available-for-sale, Debt securities | U.S. Treasury securities
Investments
Less Than 12 Months, Fair Value	9,946
Less Than 12 Months, Unrealized Losses	15
Less Than 12 Months, Fair Value		9,870
Less Than 12 Months, Unrealized Losses		22
Unrestricted available-for-sale, Debt securities | Corporate debt securities
Investments
12 Months or Greater, Fair Value	4,159
12 Months or Greater, Unrealized Losses	814
Less Than 12 Months, Fair Value		15,515
Less Than 12 Months, Unrealized Losses		691
12 Months or Greater, Fair Value		29,595
12 Months or Greater, Unrealized Losses		1,126
Equity securities | Preferred stock
Investments
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
Restricted | Short-term bond
Investments
12 Months or Greater, Fair Value	3,239
12 Months or Greater, Unrealized Losses	$ 13
12 Months or Greater, Fair Value		4,251
12 Months or Greater, Unrealized Losses		$ 13