Investments - Schedule of Available-for-Sale Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,078,817	$ 1,014,765
Gross Unrealized Gains	23,308	16,300
Gross Unrealized Losses	(6,602)	(13,587)
Fair Value	1,095,523	1,017,478
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	1,006,715	940,212
Gross Unrealized Gains	15,192	11,797
Gross Unrealized Losses	(5,089)	(10,932)
Fair Value	1,016,818	941,077
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	129,786	101,793
Gross Unrealized Gains	5,608	5,032
Gross Unrealized Losses	(416)	(984)
Fair Value	134,978	105,841
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	140,053	152,703
Gross Unrealized Gains	946	1,070
Gross Unrealized Losses	(1,942)	(2,975)
Fair Value	139,057	150,798
Corporate
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	415,817	379,727
Gross Unrealized Gains	7,490	4,382
Gross Unrealized Losses	(1,901)	(5,661)
Fair Value	421,406	378,448
Commercial mortgage and asset-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	176,952	167,967
Gross Unrealized Gains	984	906
Gross Unrealized Losses	(489)	(826)
Fair Value	177,447	168,047
Obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	62,435	64,823
Gross Unrealized Gains	37	276
Gross Unrealized Losses	(97)	(85)
Fair Value	62,375	65,014
U.S. Treasury securities and obligations guaranteed by the U.S. government
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	79,647	71,174
Gross Unrealized Gains	77	131
Gross Unrealized Losses	(244)	(185)
Fair Value	79,480	71,120
Redeemable preferred stock
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	2,025	2,025
Gross Unrealized Gains	50	0
Gross Unrealized Losses	0	(216)
Fair Value	2,075	1,809
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	72,102	74,553
Gross Unrealized Gains	8,116	4,503
Gross Unrealized Losses	(1,513)	(2,655)
Fair Value	$ 78,705	$ 76,401