Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Financial Assets
Beginning balance	$ 246,505	$ 217,642	$ 212,104	$ 173,426
Transfers into Level 3	35,462	18,030	97,116	68,483
Transfers out of Level 3	(880)	(6,755)	(19,151)	(6,755)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	(161)	(55)	(1,875)	(712)
Net unrealized investment gains (losses) included in other comprehensive income	(187)	2,882	5,748	7,666
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	(1,999)	0	(1,999)	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(13,659)	(6,802)	(26,862)	(17,166)
Ending balance	265,081	224,942	265,081	224,942
Financial Liabilities
Beginning balance	67,995	47,706	59,393	39,021
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	2,587	68	6,133	2,066
Net unrealized investment gains (losses) included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Issuances	3,752	6,710	10,538	15,194
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(1,348)	695	(3,078)	(1,102)
Ending balance	72,986	55,179	72,986	55,179
Short-term Investments
Financial Assets
Beginning balance	0	0	751	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	(751)	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	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
Municipal bonds
Financial Assets
Beginning balance	49,123	47,647	46,497	30,379
Transfers into Level 3	0	0	5,214	14,751
Transfers out of Level 3	0	0	(5,557)	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	382	(2,361)	3,540	420
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(293)	(120)	(482)	(384)
Ending balance	49,212	45,166	49,212	45,166
Corporate bonds
Financial Assets
Beginning balance	77,052	73,408	60,191	67,575
Transfers into Level 3	23,501	10,375	55,420	21,451
Transfers out of Level 3	1	(5,967)	(11,962)	(5,967)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	(1)	1	(1)	(656)
Net unrealized investment gains (losses) included in other comprehensive income	(192)	1,292	263	3,073
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	(1,999)	0	(1,999)	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(4,117)	(1,488)	(7,667)	(7,855)
Ending balance	94,245	77,621	94,245	77,621
Mortgage-Backed Securities
Financial Assets
Beginning balance	120,324	96,581	104,659	75,466
Transfers into Level 3	11,961	7,655	36,482	32,281
Transfers out of Level 3	(881)	(788)	(881)	(788)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	(160)	(56)	(1,874)	(56)
Net unrealized investment gains (losses) included in other comprehensive income	(377)	3,951	1,945	4,173
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(9,249)	(5,194)	(18,713)	(8,927)
Ending balance	121,618	102,149	121,618	102,149
Total Fixed Maturity Securities
Financial Assets
Beginning balance	246,499	217,636	211,347	173,420
Transfers into Level 3	35,462	18,030	97,116	68,483
Transfers out of Level 3	(880)	(6,755)	(18,400)	(6,755)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	(161)	(55)	(1,875)	(712)
Net unrealized investment gains (losses) included in other comprehensive income	(187)	2,882	5,748	7,666
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	(1,999)	0	(1,999)	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(13,659)	(6,802)	(26,862)	(17,166)
Ending balance	265,075	224,936	265,075	224,936
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	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	$ 6	$ 6	$ 6	$ 6