Fair Value of Financial Instruments - Transfers Between Different Fair Value Levels (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Financial Assets
Beginning balance	$ 230,257	$ 212,104
Transfers into Level 3	91,259	86,788
Transfers out of Level 3	(16,479)	(29,102)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(484)	(1,833)
Net unrealized investment gains (losses) included in OCI	(9,605)	6,713
Purchases	0	0
Issuances	0	0
Sales	(6,326)	(11,178)
Settlements	0	0
Paydowns, maturities and distributions	(40,133)	(33,235)
Ending balance	248,489	230,257
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	(7,518)	12,942
Net unrealized investment gains (losses) included in OCI	0	0
Purchases	0	0
Issuances	11,183	12,605
Sales	0	0
Settlements	0	0
Paydowns, maturities and distributions	(5,698)	(4,207)
Ending balance	78,700	80,733
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 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	0
Municipal bonds
Financial Assets
Beginning balance	49,328	46,497
Transfers into Level 3	0	5,214
Transfers out of Level 3	0	(5,557)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	0	0
Net unrealized investment gains (losses) included in OCI	(1,195)	3,977
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Paydowns, maturities and distributions	(602)	(803)
Ending balance	47,531	49,328
Corporate bonds
Financial Assets
Beginning balance	72,979	60,191
Transfers into Level 3	40,488	38,483
Transfers out of Level 3	(11,279)	(16,252)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(487)	(1)
Net unrealized investment gains (losses) included in OCI	(2,840)	661
Purchases	0	0
Issuances	0	0
Sales	(6,135)	(1,999)
Settlements	0	0
Paydowns, maturities and distributions	(11,984)	(8,104)
Ending balance	80,742	72,979
Other mortgage-backed securities
Financial Assets
Beginning balance	107,944	104,659
Transfers into Level 3	50,771	43,091
Transfers out of Level 3	(5,200)	(6,542)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	0	(1,832)
Net unrealized investment gains (losses) included in OCI	(5,570)	2,075
Purchases	0	0
Issuances	0	0
Sales	(187)	(9,179)
Settlements	0	0
Paydowns, maturities and distributions	(27,547)	(24,328)
Ending balance	120,211	107,944
Total
Financial Assets
Beginning balance	230,251	211,347
Transfers into Level 3	91,259	86,788
Transfers out of Level 3	(16,479)	(28,351)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(487)	(1,833)
Net unrealized investment gains (losses) included in OCI	(9,605)	6,713
Purchases	0	0
Issuances	0	0
Sales	(6,322)	(11,178)
Settlements	0	0
Paydowns, maturities and distributions	(40,133)	(33,235)
Ending balance	248,484	230,251
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 investment gains (losses) included in net income related to financial assets	3	0
Net unrealized investment gains (losses) included in OCI	0	0
Purchases	0	0
Issuances	0	0
Sales	(4)	0
Settlements	0	0
Paydowns, maturities and distributions	0	0
Ending balance	$ 5	$ 6

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