Fair Value Measurement - Level 3 Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation:
Beginning Balance	$ 17,820	$ 15,257
Total gains (losses) realized and unrealized:
Included in other comprehensive income	(300)	586
Purchases	15,643	17,153
Sales	(5,749)	(7,249)
Transfers in	2,320	2,818
Transfers out	(10,343)	(10,078)
Ending Balance	19,098	17,820
Change in unrealized gains (losses) included in earnings for the above period for Level 3 assets held at period-end	0	0
State and municipal bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation:
Beginning Balance	0	0
Total gains (losses) realized and unrealized:
Included in other comprehensive income	0	0
Purchases	0	0
Sales	0	(410)
Transfers in	0	900
Transfers out	0	0
Ending Balance		0
Change in unrealized gains (losses) included in earnings for the above period for Level 3 assets held at period-end	0	0
Corporate debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation:
Beginning Balance	14,810	14,500
Total gains (losses) realized and unrealized:
Included in other comprehensive income	(369)	531
Purchases	13,016	8,900
Sales	(4,837)	(3,837)
Transfers in	999	0
Transfers out	(9,766)	(5,116)
Ending Balance	13,703	14,810
Change in unrealized gains (losses) included in earnings for the above period for Level 3 assets held at period-end	0	0
Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation:
Beginning Balance	3,007	757
Total gains (losses) realized and unrealized:
Included in other comprehensive income	(71)	8
Purchases	2,627	6,500
Sales	0	(1,452)
Transfers in	0	1,000
Transfers out	(577)	(3,806)
Ending Balance	4,986	3,007
Change in unrealized gains (losses) included in earnings for the above period for Level 3 assets held at period-end	0	0
Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation:
Beginning Balance	3	0
Total gains (losses) realized and unrealized:
Included in other comprehensive income	140	47
Purchases	0	1,753
Sales	(912)	(1,550)
Transfers in	1,321	918
Transfers out	0	(1,156)
Ending Balance	409	3
Change in unrealized gains (losses) included in earnings for the above period for Level 3 assets held at period-end	0	0
Net investment income
Total gains (losses) realized and unrealized:
Included in earnings	(163)	(102)
Net investment income | State and municipal bonds
Total gains (losses) realized and unrealized:
Included in earnings	0	0
Net investment income | Corporate debt
Total gains (losses) realized and unrealized:
Included in earnings	(163)	(93)
Net investment income | Asset-backed Securities
Total gains (losses) realized and unrealized:
Included in earnings	0	0
Net investment income | Other investments
Total gains (losses) realized and unrealized:
Included in earnings	0	(9)
Net realized investment gains (losses)
Total gains (losses) realized and unrealized:
Included in earnings	(130)	(565)
Net realized investment gains (losses) | State and municipal bonds
Total gains (losses) realized and unrealized:
Included in earnings	0	(490)
Net realized investment gains (losses) | Corporate debt
Total gains (losses) realized and unrealized:
Included in earnings	13	(75)
Net realized investment gains (losses) | Asset-backed Securities
Total gains (losses) realized and unrealized:
Included in earnings	0	0
Net realized investment gains (losses) | Other investments
Total gains (losses) realized and unrealized:
Included in earnings	$ (143)	$ 0