Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Financial Assets
Beginning balance	$ 243,868	$ 248,757	$ 230,257	$ 212,104
Transfers into Level 3	48,031	11,483	73,631	61,654
Transfers out of Level 3	(15,509)	(11,410)	(15,509)	(18,271)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	(246)	(1,714)	(243)	(1,714)
Net unrealized investment gains (losses) included in other comprehensive income	1,356	4,739	(110)	5,935
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	(3)	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(5,849)	(5,350)	(16,372)	(13,203)
Ending balance	271,651	246,505	271,651	246,505
Financial Liabilities
Beginning balance	78,486	64,261	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	(1,291)	1,238	(3,513)	3,546
Net unrealized investment gains (losses) included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Issuances	2,107	3,397	3,439	6,786
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(1,514)	(901)	(2,871)	(1,730)
Ending balance	77,788	67,995	77,788	67,995
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	243,862	248,751	230,251	211,347
Transfers into Level 3	48,031	11,483	73,631	61,654
Transfers out of Level 3	(15,509)	(11,410)	(15,509)	(17,520)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	(246)	(1,714)	(246)	(1,714)
Net unrealized investment gains (losses) included in other comprehensive income	1,356	4,739	(110)	5,935
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(5,849)	(5,350)	(16,372)	(13,203)
Ending balance	271,645	246,499	271,645	246,499
Municipal bonds
Financial Assets
Beginning balance	49,748	53,462	49,328	46,497
Transfers into Level 3	0	0	0	5,214
Transfers out of Level 3	0	(5,557)	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	397	1,287	840	3,158
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(224)	(69)	(247)	(189)
Ending balance	49,921	49,123	49,921	49,123
Corporate bonds
Financial Assets
Beginning balance	78,780	82,495	72,979	60,191
Transfers into Level 3	29,709	2,001	40,487	31,919
Transfers out of Level 3	(11,279)	(5,853)	(11,279)	(11,963)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	(246)	0	(246)	0
Net unrealized investment gains (losses) included in other comprehensive income	(700)	359	(1,587)	455
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(3,601)	(1,950)	(7,691)	(3,550)
Ending balance	92,663	77,052	92,663	77,052
Mortgage-Backed Securities
Financial Assets
Beginning balance	115,334	112,794	107,944	104,659
Transfers into Level 3	18,322	9,482	33,144	24,521
Transfers out of Level 3	(4,230)	0	(4,230)	0
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	(1,714)	0	(1,714)
Net unrealized investment gains (losses) included in other comprehensive income	1,659	3,093	637	2,322
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(2,024)	(3,331)	(8,434)	(9,464)
Ending balance	129,061	120,324	129,061	120,324
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