Investments - Investments Held in a Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
U.S. Treasury obligations
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 110,788	$ 79,833
Unrealized Loss	1,181	911
Less than 12 months, Fair Value	67,135	79,833
Less than 12 months, Unrealized loss	554	911
12 months or longer, Fair Value	43,653	0
12 months or longer, Unrealized Loss	627	0
U.S. Government-sponsored enterprise obligations
Schedule of Available-for-sale Securities [Line Items]
Fair Value	17,032	11,746
Unrealized Loss	206	191
Less than 12 months, Fair Value	10,182	11,746
Less than 12 months, Unrealized loss	64	191
12 months or longer, Fair Value	6,850	0
12 months or longer, Unrealized Loss	142	0
State and municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	23,122	224,884
Unrealized Loss	419	6,952
Less than 12 months, Fair Value	15,168	219,276
Less than 12 months, Unrealized loss	102	6,444
12 months or longer, Fair Value	7,954	5,608
12 months or longer, Unrealized Loss	317	508
Corporate debt
Schedule of Available-for-sale Securities [Line Items]
Fair Value	487,578	469,632
Unrealized Loss	5,707	8,480
Less than 12 months, Fair Value	365,541	424,721
Less than 12 months, Unrealized loss	2,730	5,662
12 months or longer, Fair Value	122,037	44,911
12 months or longer, Unrealized Loss	2,977	2,818
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	109,659	103,680
Unrealized Loss	1,335	2,046
Less than 12 months, Fair Value	64,121	100,542
Less than 12 months, Unrealized loss	402	1,982
12 months or longer, Fair Value	45,538	3,138
12 months or longer, Unrealized Loss	933	64
Agency commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	4,423	4,579
Unrealized Loss	108	143
Less than 12 months, Fair Value	2,458	4,192
Less than 12 months, Unrealized loss	34	114
12 months or longer, Fair Value	1,965	387
12 months or longer, Unrealized Loss	74	29
Other commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	12,878	9,822
Unrealized Loss	134	137
Less than 12 months, Fair Value	7,939	9,179
Less than 12 months, Unrealized loss	82	134
12 months or longer, Fair Value	4,939	643
12 months or longer, Unrealized Loss	52	3
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	85,358	44,343
Unrealized Loss	466	267
Less than 12 months, Fair Value	70,924	39,079
Less than 12 months, Unrealized loss	346	256
12 months or longer, Fair Value	14,434	5,264
12 months or longer, Unrealized Loss	120	11
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	850,838	948,519
Unrealized Loss	9,556	19,127
Less than 12 months, Fair Value	603,468	888,568
Less than 12 months, Unrealized loss	4,314	15,694
12 months or longer, Fair Value	247,370	59,951
12 months or longer, Unrealized Loss	$ 5,242	$ 3,433