Investments - Investments Held in a Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
U.S. Treasury obligations
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 79,833	$ 66,685
Unrealized Loss	911	658
Less than 12 months, Fair Value	79,833	61,869
Less than 12 months, Unrealized loss	911	591
12 months or longer, Fair Value	0	4,816
12 months or longer, Unrealized Loss	0	67
U.S. Government-sponsored enterprise obligations
Schedule of Available-for-sale Securities [Line Items]
Fair Value	11,746	6,819
Unrealized Loss	191	49
Less than 12 months, Fair Value	11,746	6,819
Less than 12 months, Unrealized loss	191	49
12 months or longer, Fair Value	0	0
12 months or longer, Unrealized Loss	0	0
State and municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	224,884	46,193
Unrealized Loss	6,952	823
Less than 12 months, Fair Value	219,276	36,822
Less than 12 months, Unrealized loss	6,444	703
12 months or longer, Fair Value	5,608	9,371
12 months or longer, Unrealized Loss	508	120
Corporate debt
Schedule of Available-for-sale Securities [Line Items]
Fair Value	469,632	622,991
Unrealized Loss	8,480	29,162
Less than 12 months, Fair Value	424,721	555,097
Less than 12 months, Unrealized loss	5,662	15,691
12 months or longer, Fair Value	44,911	67,894
12 months or longer, Unrealized Loss	2,818	13,471
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	103,680	87,567
Unrealized Loss	2,046	1,311
Less than 12 months, Fair Value	100,542	78,961
Less than 12 months, Unrealized loss	1,982	1,095
12 months or longer, Fair Value	3,138	8,606
12 months or longer, Unrealized Loss	64	216
Agency commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	4,579	409
Unrealized Loss	143	26
Less than 12 months, Fair Value	4,192	0
Less than 12 months, Unrealized loss	114	0
12 months or longer, Fair Value	387	409
12 months or longer, Unrealized Loss	29	26
Other commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	9,822	15,960
Unrealized Loss	137	203
Less than 12 months, Fair Value	9,179	12,635
Less than 12 months, Unrealized loss	134	170
12 months or longer, Fair Value	643	3,325
12 months or longer, Unrealized Loss	3	33
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	44,343	79,637
Unrealized Loss	267	247
Less than 12 months, Fair Value	39,079	74,150
Less than 12 months, Unrealized loss	256	237
12 months or longer, Fair Value	5,264	5,487
12 months or longer, Unrealized Loss	11	10
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	948,519	926,261
Unrealized Loss	19,127	32,479
Less than 12 months, Fair Value	888,568	826,353
Less than 12 months, Unrealized loss	15,694	18,536
12 months or longer, Fair Value	59,951	99,908
12 months or longer, Unrealized Loss	$ 3,433	$ 13,943