Investments - Schedule of Securities in Unrealized Loss Position (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Securities	Dec. 31, 2016 USD ($) Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than 12 months, fair value	$ 427,430	$ 380,199
Available-for-sale securities, continuous unrealized loss position, less than 12 months, aggregate losses	(3,659)	(5,207)
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, fair value	95,841	62,955
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	(2,283)	(2,568)
Fixed-maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than 12 months, fair value	426,354	376,020
Available-for-sale securities, continuous unrealized loss position, less than 12 months, aggregate losses	(3,643)	(4,938)
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, fair value	95,671	61,103
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	(2,279)	(2,324)
U.S. Government and Agencies [Member] | Fixed-maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than 12 months, fair value	4,754	3,668
Available-for-sale securities, continuous unrealized loss position, less than 12 months, aggregate losses	$ (34)	$ (24)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, less than one year | Securities	5	4
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, fair value	$ 2,975
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	$ (13)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year | Securities	3
Foreign Government [Member] | Fixed-maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than 12 months, fair value	$ 40,287	$ 34,538
Available-for-sale securities, continuous unrealized loss position, less than 12 months, aggregate losses	$ (465)	$ (526)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, less than one year | Securities	45	36
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, fair value	$ 7,102	$ 3,048
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	$ (183)	$ (161)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year | Securities	7	3
States and Political Subdivisions [Member] | Fixed-maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than 12 months, fair value	$ 7,369	$ 8,902
Available-for-sale securities, continuous unrealized loss position, less than 12 months, aggregate losses	$ (43)	$ (129)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, less than one year | Securities	7	12
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, fair value	$ 6,267
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	$ (98)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year | Securities	7
Corporates [Member] | Fixed-maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than 12 months, fair value	$ 247,613	$ 232,070
Available-for-sale securities, continuous unrealized loss position, less than 12 months, aggregate losses	$ (2,323)	$ (3,484)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, less than one year | Securities	216	225
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, fair value	$ 39,767	$ 45,471
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	$ (1,332)	$ (2,045)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year | Securities	43	51
Residential Mortgage-backed Securities [Member] | Fixed-maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than 12 months, fair value	$ 33,610	$ 15,232
Available-for-sale securities, continuous unrealized loss position, less than 12 months, aggregate losses	$ (263)	$ (92)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, less than one year | Securities	16	9
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, fair value	$ 2,592	$ 3,606
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	$ (34)	$ (28)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year | Securities	8	9
Commercial Mortgage-backed Securities [Member] | Fixed-maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than 12 months, fair value	$ 60,116	$ 33,335
Available-for-sale securities, continuous unrealized loss position, less than 12 months, aggregate losses	$ (394)	$ (423)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, less than one year | Securities	52	33
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, fair value	$ 22,149	$ 7,663
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	$ (516)	$ (47)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year | Securities	25	11
Other Asset-backed Securities [Member] | Fixed-maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than 12 months, fair value	$ 32,605	$ 48,275
Available-for-sale securities, continuous unrealized loss position, less than 12 months, aggregate losses	$ (121)	$ (260)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, less than one year | Securities	33	45
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, fair value	$ 14,819	$ 1,315
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	$ (103)	$ (43)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year | Securities	19	3
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than 12 months, fair value	$ 1,076	$ 4,179
Available-for-sale securities, continuous unrealized loss position, less than 12 months, aggregate losses	$ (16)	$ (269)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, less than one year | Securities	4	12
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, fair value	$ 170	$ 1,852
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	$ (4)	$ (244)
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year | Securities	2	8