Fair Value of Financial Instruments - Transfers Between Different Fair Value Levels (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Financial Assets
Beginning balance	$ 212,104	$ 173,426
Transfers into Level 3	86,788	85,150
Transfers out of Level 3	(29,102)	(21,028)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	(1,833)	(1,889)
Net unrealized investment gains (losses) included in OCI	6,713	4,700
Purchases	0	0
Issuances	0	0
Sales	(11,178)	0
Settlements	0	0
Paydowns, maturities and distributions	(33,235)	(28,255)
Ending balance	230,257	212,104
Financial Liabilities
Beginning balance	59,393	39,021
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	12,942	5,011
Net unrealized investment gains (losses) included in OCI	0	0
Purchases	0	0
Issuances	12,605	17,113
Sales	0	0
Settlements	0	0
Paydowns, maturities and distributions	(4,207)	(1,752)
Ending balance	80,733	59,393
Short-term Investments
Financial Assets
Beginning balance	751	0
Transfers into Level 3	0	751
Transfers out of Level 3	(751)	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 OCI	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Paydowns, maturities and distributions	0	0
Ending balance	0	751
Municipal bonds
Financial Assets
Beginning balance	46,497	30,379
Transfers into Level 3	5,214	17,710
Transfers out of Level 3	(5,557)	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 OCI	3,977	(990)
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Paydowns, maturities and distributions	(803)	(602)
Ending balance	49,328	46,497
Corporate bonds
Financial Assets
Beginning balance	60,191	67,575
Transfers into Level 3	38,483	27,561
Transfers out of Level 3	(16,252)	(14,334)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	(1)	(1,833)
Net unrealized investment gains (losses) included in OCI	661	(205)
Purchases	0	0
Issuances	0	0
Sales	(1,999)	0
Settlements	0	0
Paydowns, maturities and distributions	(8,104)	(18,573)
Ending balance	72,979	60,191
Other mortgage-backed securities
Financial Assets
Beginning balance	104,659	75,466
Transfers into Level 3	43,091	39,128
Transfers out of Level 3	(6,542)	(6,694)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	(1,832)	(56)
Net unrealized investment gains (losses) included in OCI	2,075	5,895
Purchases	0	0
Issuances	0	0
Sales	(9,179)	0
Settlements	0	0
Paydowns, maturities and distributions	(24,328)	(9,080)
Ending balance	107,944	104,659
Total fixed maturity securities
Financial Assets
Beginning balance	211,347	173,420
Transfers into Level 3	86,788	84,399
Transfers out of Level 3	(28,351)	(21,028)
Total gains or losses
Net realized investment gains (losses) included in net income related to financial assets	(1,833)	(1,889)
Net unrealized investment gains (losses) included in OCI	6,713	4,700
Purchases	0	0
Issuances	0	0
Sales	(11,178)	0
Settlements	0	0
Paydowns, maturities and distributions	(33,235)	(28,255)
Ending balance	230,251	211,347
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	0	0
Net unrealized investment gains (losses) included in OCI	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Paydowns, maturities and distributions	0	0
Ending balance	$ 6	$ 6

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