Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Financial Assets
Beginning balance	$ 230,257	$ 212,104
Transfers into Level 3	25,600	50,171
Transfers out of Level 3	0	(6,861)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	3	0
Net unrealized investment gains (losses) included in other comprehensive income	(1,466)	1,196
Purchases	0	0
Issuances	0	0
Sales	(3)	0
Settlements	0	0
Paydowns, maturities and distributions	(10,523)	(7,853)
Ending balance	243,868	248,757
Financial Liabilities
Beginning balance	80,733	59,393
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Total gains or losses
Net realized gains (losses) included in net income related to financial liabilities	(2,222)	2,308
Net unrealized investment gains (losses) included in other comprehensive income		0
Purchases		0
Issuances	1,332	3,389
Sales		0
Settlements		0
Paydowns, maturities and distributions	(1,357)	(829)
Ending balance	78,486	64,261
Short-term Investments
Financial Assets
Beginning balance	0	751
Transfers into Level 3	0	0
Transfers out of Level 3	0	(751)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	0
Net unrealized investment gains (losses) included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Paydowns, maturities and distributions	0	0
Ending balance	0	0
Municipal bonds
Financial Assets
Beginning balance	49,328	46,497
Transfers into Level 3	0	5,214
Transfers out of Level 3	0	0
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	0
Net unrealized investment gains (losses) included in other comprehensive income	443	1,871
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Paydowns, maturities and distributions	(23)	(120)
Ending balance	49,748	53,462
Corporate bonds
Financial Assets
Beginning balance	72,979	60,191
Transfers into Level 3	10,778	29,918
Transfers out of Level 3	0	(6,110)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	0
Net unrealized investment gains (losses) included in other comprehensive income	(887)	96
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Paydowns, maturities and distributions	(4,090)	(1,600)
Ending balance	78,780	82,495
Mortgage-Backed Securities
Financial Assets
Beginning balance	107,944	104,659
Transfers into Level 3	14,822	15,039
Transfers out of Level 3	0	0
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	0
Net unrealized investment gains (losses) included in other comprehensive income	(1,022)	(771)
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Paydowns, maturities and distributions	(6,410)	(6,133)
Ending balance	115,334	112,794
Total Fixed Maturity Securities
Financial Assets
Beginning balance	230,251	211,347
Transfers into Level 3	25,600	50,171
Transfers out of Level 3	0	(6,110)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	0	0
Net unrealized investment gains (losses) included in other comprehensive income	(1,466)	1,196
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Paydowns, maturities and distributions	(10,523)	(7,853)
Ending balance	243,862	248,751
Equity securities
Financial Assets
Beginning balance	6	6
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	3	0
Net unrealized investment gains (losses) included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	(3)	0
Settlements	0	0
Paydowns, maturities and distributions	0	0
Ending balance	$ 6	$ 6

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Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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