Investments Available-for-Sale Securities in a Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Estimated Fair Value	$ 246,411	$ 89,168
Gross unrealized holding losses, less than 12 months	(3,489)	(558)
12 Months or Longer, Estimated Fair Value	117,446	125,043
12 Months or Longer, Gross Unrealized Holding Losses	(5,394)	(1,917)
Total Estimated Fair Value	363,857	214,211
Total Gross Unrealized Holding Losses	(8,883)	(2,475)
Total fixed maturities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Estimated Fair Value		78,389
Gross unrealized holding losses, less than 12 months		(327)
12 Months or Longer, Estimated Fair Value		125,043
12 Months or Longer, Gross Unrealized Holding Losses		(1,917)
Total Estimated Fair Value	363,900	203,432
Total Gross Unrealized Holding Losses	(8,900)	(2,244)
U.S. Treasury securities and obligations of U.S. government agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Estimated Fair Value	0	3,497
Gross unrealized holding losses, less than 12 months	0	(2)
12 Months or Longer, Estimated Fair Value	497	5,488
12 Months or Longer, Gross Unrealized Holding Losses	(2)	(12)
Total Estimated Fair Value	497	8,985
Total Gross Unrealized Holding Losses	(2)	(14)
Obligations of states, municipalities and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Estimated Fair Value	65,685	7,258
Gross unrealized holding losses, less than 12 months	(1,514)	(36)
12 Months or Longer, Estimated Fair Value	36,359	38,143
12 Months or Longer, Gross Unrealized Holding Losses	(1,487)	(376)
Total Estimated Fair Value	102,044	45,401
Total Gross Unrealized Holding Losses	(3,001)	(412)
Corporate and other securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Estimated Fair Value	64,046	30,944
Gross unrealized holding losses, less than 12 months	(761)	(98)
12 Months or Longer, Estimated Fair Value	11,201	13,444
12 Months or Longer, Gross Unrealized Holding Losses	(180)	(74)
Total Estimated Fair Value	75,247	44,388
Total Gross Unrealized Holding Losses	(941)	(172)
Asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Estimated Fair Value	84,051	27,609
Gross unrealized holding losses, less than 12 months	(925)	(108)
12 Months or Longer, Estimated Fair Value	13,965	10,706
12 Months or Longer, Gross Unrealized Holding Losses	(408)	(160)
Total Estimated Fair Value	98,016	38,315
Total Gross Unrealized Holding Losses	(1,333)	(268)
Residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Estimated Fair Value	32,629	9,081
Gross unrealized holding losses, less than 12 months	(289)	(83)
12 Months or Longer, Estimated Fair Value	55,424	57,262
12 Months or Longer, Gross Unrealized Holding Losses	(3,317)	(1,295)
Total Estimated Fair Value	88,053	66,343
Total Gross Unrealized Holding Losses	$ (3,606)	(1,378)
Total equity securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Estimated Fair Value		10,779
Gross unrealized holding losses, less than 12 months		(231)
12 Months or Longer, Estimated Fair Value		0
12 Months or Longer, Gross Unrealized Holding Losses		0
Total Estimated Fair Value		10,779
Total Gross Unrealized Holding Losses		(231)
Exchange traded funds [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Estimated Fair Value		130
Gross unrealized holding losses, less than 12 months		0
12 Months or Longer, Estimated Fair Value		0
12 Months or Longer, Gross Unrealized Holding Losses		0
Total Estimated Fair Value		130
Total Gross Unrealized Holding Losses		0
Nonredeemable preferred stock [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Estimated Fair Value		10,649
Gross unrealized holding losses, less than 12 months		(231)
12 Months or Longer, Estimated Fair Value		0
12 Months or Longer, Gross Unrealized Holding Losses		0
Total Estimated Fair Value		10,649
Total Gross Unrealized Holding Losses		$ (231)