Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Financial Assets
Beginning balance	$ 248,757	$ 200,391	$ 212,104	$ 173,426
Transfers into Level 3	11,483	18,001	61,654	50,453
Transfers out of Level 3	(11,410)	0	(18,271)	0
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	(1,714)	(657)	(1,714)	(657)
Net unrealized investment gains (losses) included in other comprehensive income	4,739	2,919	5,935	4,784
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(5,350)	(3,012)	(13,203)	(10,364)
Ending balance	246,505	217,642	246,505	217,642
Financial Liabilities
Beginning balance	64,261	42,085	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	1,238	1,324	3,546	1,998
Net unrealized investment gains (losses) included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Issuances	3,397	4,993	6,786	8,484
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(901)	(696)	(1,730)	(1,797)
Ending balance	67,995	47,706	67,995	47,706
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	53,462	46,493	46,497	30,379
Transfers into Level 3	0	0	5,214	14,751
Transfers out of Level 3	(5,557)	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	1,287	1,297	3,158	2,781
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(69)	(143)	(189)	(264)
Ending balance	49,123	47,647	49,123	47,647
Corporate bonds
Financial Assets
Beginning balance	82,495	70,071	60,191	67,575
Transfers into Level 3	2,001	5,017	31,919	11,076
Transfers out of Level 3	(5,853)	0	(11,963)	0
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	(657)	0	(657)
Net unrealized investment gains (losses) included in other comprehensive income	359	1,393	455	1,781
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(1,950)	(2,416)	(3,550)	(6,367)
Ending balance	77,052	73,408	77,052	73,408
Mortgage-Backed Securities
Financial Assets
Beginning balance	112,794	83,821	104,659	75,466
Transfers into Level 3	9,482	12,984	24,521	24,626
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	(1,714)	0	(1,714)	0
Net unrealized investment gains (losses) included in other comprehensive income	3,093	229	2,322	222
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(3,331)	(453)	(9,464)	(3,733)
Ending balance	120,324	96,581	120,324	96,581
Total Fixed Maturity Securities
Financial Assets
Beginning balance	248,751	200,385	211,347	173,420
Transfers into Level 3	11,483	18,001	61,654	50,453
Transfers out of Level 3	(11,410)	0	(17,520)	0
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	(1,714)	(657)	(1,714)	(657)
Net unrealized investment gains (losses) included in other comprehensive income	4,739	2,919	5,935	4,784
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(5,350)	(3,012)	(13,203)	(10,364)
Ending balance	246,499	217,636	246,499	217,636
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

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