Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financial Assets
Beginning balance	$ 271,651	$ 246,505	$ 230,257	$ 212,104
Transfers into Level 3	17,030	35,462	90,661	97,116
Transfers out of Level 3	(970)	(880)	(16,479)	(19,151)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	(161)	(243)	(1,875)
Net unrealized investment gains (losses) included in other comprehensive income	(6,527)	(187)	(6,637)	5,748
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	(187)	(1,999)	(190)	(1,999)
Settlements	0	0	0	0
Paydowns, maturities and distributions	(19,513)	(13,659)	(35,885)	(26,862)
Ending balance	261,484	265,081	261,484	265,081
Financial Liabilities
Beginning balance	77,788	67,995	80,733	59,393
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	2,205	2,587	(1,308)	6,133
Net unrealized investment gains (losses) included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Issuances	3,940	3,752	7,379	10,538
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	1,668	(1,348)	4,539	(3,078)
Ending balance	82,265	72,986	82,265	72,986
Short-term Investments
Financial Assets
Beginning balance	0	0	0	751
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	(751)
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	0	0	0	0
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	0	0	0	0
Ending balance	0	0	0	0
Total Fixed Maturity Securities
Financial Assets
Beginning balance	271,645	246,499	230,251	211,347
Transfers into Level 3	17,030	35,462	90,661	97,116
Transfers out of Level 3	(970)	(880)	(16,479)	(18,400)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	(161)	(246)	(1,875)
Net unrealized investment gains (losses) included in other comprehensive income	(6,527)	(187)	(6,637)	5,748
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	(187)	(1,999)	(187)	(1,999)
Settlements	0	0	0	0
Paydowns, maturities and distributions	(19,513)	(13,659)	(35,885)	(26,862)
Ending balance	261,478	265,075	261,478	265,075
Municipal bonds
Financial Assets
Beginning balance	49,921	49,123	49,328	46,497
Transfers into Level 3	0	0	0	5,214
Transfers out of Level 3	0	0	0	(5,557)
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	(471)	382	369	3,540
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(121)	(293)	(368)	(482)
Ending balance	49,329	49,212	49,329	49,212
Corporate bonds
Financial Assets
Beginning balance	92,663	77,052	72,979	60,191
Transfers into Level 3	0	23,501	40,487	55,420
Transfers out of Level 3	0	1	(11,279)	(11,962)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	(1)	(246)	(1)
Net unrealized investment gains (losses) included in other comprehensive income	128	(192)	(1,459)	263
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	(1,999)	0	(1,999)
Settlements	0	0	0	0
Paydowns, maturities and distributions	(3,926)	(4,117)	(11,617)	(7,667)
Ending balance	88,865	94,245	88,865	94,245
Mortgage-Backed Securities
Financial Assets
Beginning balance	129,061	120,324	107,944	104,659
Transfers into Level 3	17,030	11,961	50,174	36,482
Transfers out of Level 3	(970)	(881)	(5,200)	(881)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	(160)	0	(1,874)
Net unrealized investment gains (losses) included in other comprehensive income	(6,184)	(377)	(5,547)	1,945
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	(187)	0	(187)	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(15,466)	(9,249)	(23,900)	(18,713)
Ending balance	123,284	121,618	123,284	121,618
Equity securities
Financial Assets
Beginning balance	6	6	6	6
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	3	0
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	(3)	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	0	0	0	0
Ending balance	$ 6	$ 6	$ 6	$ 6