Investments - Schedule of Available-for-Sale Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,135,366	$ 1,083,980
Gross Unrealized Gains	14,683	22,394
Gross Unrealized Losses	(20,841)	(7,754)
Fair Value	1,129,208	1,098,620
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	1,054,902	1,008,662
Gross Unrealized Gains	7,838	14,564
Gross Unrealized Losses	(19,489)	(7,128)
Fair Value	1,043,251	1,016,098
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	137,478	139,382
Gross Unrealized Gains	4,029	5,587
Gross Unrealized Losses	(1,782)	(603)
Fair Value	139,725	144,366
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	186,279	160,379
Gross Unrealized Gains	509	723
Gross Unrealized Losses	(5,690)	(2,441)
Fair Value	181,098	158,661
Corporate
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	445,058	408,857
Gross Unrealized Gains	3,054	7,503
Gross Unrealized Losses	(8,486)	(2,639)
Fair Value	439,626	413,721
Commercial mortgage and asset-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	192,941	182,595
Gross Unrealized Gains	173	714
Gross Unrealized Losses	(2,310)	(698)
Fair Value	190,804	182,611
Obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	7,804	35,948
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(29)	(101)
Fair Value	7,775	35,847
U.S. Treasury securities and obligations guaranteed by the U.S. government
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	83,317	79,476
Gross Unrealized Gains	73	37
Gross Unrealized Losses	(1,171)	(639)
Fair Value	82,219	78,874
Redeemable preferred stock
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	2,025	2,025
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(21)	(7)
Fair Value	2,004	2,018
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	80,464	75,318
Gross Unrealized Gains	6,845	7,830
Gross Unrealized Losses	(1,352)	(626)
Fair Value	$ 85,957	$ 82,522