Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Financial Assets
Beginning balance	$ 266,033	$ 243,868	$ 248,489	$ 230,257
Transfers into Level 3	2,872	48,031	27,880	73,631
Transfers out of Level 3	(4,876)	(15,509)	(4,876)	(15,509)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	(246)	0	(243)
Net unrealized investment gains (losses) included in other comprehensive income	4,231	1,356	6,972	(110)
Purchases	1,566	0	1,566	0
Issuances	0	0	0	0
Sales	(607)	0	(607)	(3)
Settlements	0	0	0	0
Paydowns, maturities and distributions	(14,506)	(5,849)	(24,711)	(16,372)
Ending balance	254,713	271,651	254,713	271,651
Financial Liabilities
Beginning balance	84,629	78,486	78,700	80,733
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Total gains or losses
Net realized gains (losses) included in net income related to financial liabilities	371	(1,291)	4,705	(3,513)
Net unrealized investment gains (losses) included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Issuances	2,431	2,107	5,449	3,439
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(1,470)	(1,514)	(2,893)	(2,871)
Ending balance	85,961	77,788	85,961	77,788
Total Fixed Maturity Securities
Financial Assets
Beginning balance	266,028	243,862	248,484	230,251
Transfers into Level 3	2,808	48,031	27,816	73,631
Transfers out of Level 3	(4,876)	(15,509)	(4,876)	(15,509)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	(246)	0	(246)
Net unrealized investment gains (losses) included in other comprehensive income	4,231	1,356	6,972	(110)
Purchases	1,566	0	1,566	0
Issuances	0	0	0	0
Sales	(607)	0	(607)	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(14,506)	(5,849)	(24,711)	(16,372)
Ending balance	254,644	271,645	254,644	271,645
Municipal bonds
Financial Assets
Beginning balance	47,756	49,748	47,531	49,328
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	0	0	0
Net unrealized investment gains (losses) included in other comprehensive income	(537)	397	(193)	840
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(235)	(224)	(354)	(247)
Ending balance	46,984	49,921	46,984	49,921
Corporate bonds
Financial Assets
Beginning balance	82,482	78,780	80,742	72,979
Transfers into Level 3	2,808	29,709	5,882	40,487
Transfers out of Level 3	(4,876)	(11,279)	(4,876)	(11,279)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	(246)	0	(246)
Net unrealized investment gains (losses) included in other comprehensive income	1,961	(700)	4,510	(1,587)
Purchases	1,566	0	1,566	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(4,719)	(3,601)	(8,602)	(7,691)
Ending balance	79,222	92,663	79,222	92,663
Mortgage-Backed Securities
Financial Assets
Beginning balance	135,790	115,334	120,211	107,944
Transfers into Level 3	0	18,322	21,934	33,144
Transfers out of Level 3	0	(4,230)	0	(4,230)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	0	0	0
Net unrealized investment gains (losses) included in other comprehensive income	2,807	1,659	2,655	637
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	(607)	0	(607)	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(9,552)	(2,024)	(15,755)	(8,434)
Ending balance	128,438	129,061	128,438	129,061
Equity securities
Financial Assets
Beginning balance	5	6	5	6
Transfers into Level 3	64	0	64	0
Transfers out of Level 3	0	0	0	0
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	0	0	3
Net unrealized investment gains (losses) included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	(3)
Settlements	0	0	0	0
Paydowns, maturities and distributions	0	0	0	0
Ending balance	$ 69	$ 6	$ 69	$ 6

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