Investments, Available-for-sale Securities in a Loss Position (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Rate	Dec. 31, 2016 USD ($) Rate	Dec. 31, 2015 USD ($)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, estimated fair value	$ 89,168	$ 210,558
Less than 12 months, gross unrealized holding losses	(558)	(4,758)
12 Months or longer, estimated fair value	125,043	29,522
12 Months or longer, gross unrealized holding losses	(1,917)	(401)
Total, estimated fair value	214,211	240,080
Total, gross unrealized holding losses	(2,475)	(5,159)
OTTIs from securities	0	276	$ 0
Other-than-temporary impairment losses	0	(276)	$ 0
Fixed Maturities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, estimated fair value	78,389	210,170
Less than 12 months, gross unrealized holding losses	(327)	(4,756)
12 Months or longer, estimated fair value	125,043	29,522
12 Months or longer, gross unrealized holding losses	(1,917)	(401)
Total, estimated fair value	203,432	239,692
Total, gross unrealized holding losses	$ (2,244)	$ (5,157)
Number of securities held	195	231
Number of available-for-sale securities in unrealized loss positions, greater than one year	126	24
Percentage of securities rated A- or better | Rate	91.10%	92.60%
Other-than-temporary impairment losses		$ 0
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, estimated fair value	$ 3,497	8,980
Less than 12 months, gross unrealized holding losses	(2)	(16)
12 Months or longer, estimated fair value	5,488	0
12 Months or longer, gross unrealized holding losses	(12)	0
Total, estimated fair value	8,985	8,980
Total, gross unrealized holding losses	(14)	(16)
Obligations of States, Municipalities and Political Subdivisions [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, estimated fair value	7,258	70,727
Less than 12 months, gross unrealized holding losses	(36)	(2,960)
12 Months or longer, estimated fair value	38,143	0
12 Months or longer, gross unrealized holding losses	(376)	0
Total, estimated fair value	45,401	70,727
Total, gross unrealized holding losses	(412)	(2,960)
Corporate and Other Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, estimated fair value	30,944	50,274
Less than 12 months, gross unrealized holding losses	(98)	(145)
12 Months or longer, estimated fair value	13,444	12,375
12 Months or longer, gross unrealized holding losses	(74)	(88)
Total, estimated fair value	44,388	62,649
Total, gross unrealized holding losses	(172)	(233)
Asset-backed Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, estimated fair value	27,609	14,750
Less than 12 months, gross unrealized holding losses	(108)	(232)
12 Months or longer, estimated fair value	10,706	9,961
12 Months or longer, gross unrealized holding losses	(160)	(32)
Total, estimated fair value	38,315	24,711
Total, gross unrealized holding losses	(268)	(264)
Residential Mortgage-backed Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, estimated fair value	9,081	65,439
Less than 12 months, gross unrealized holding losses	(83)	(1,403)
12 Months or longer, estimated fair value	57,262	7,186
12 Months or longer, gross unrealized holding losses	(1,295)	(281)
Total, estimated fair value	66,343	72,625
Total, gross unrealized holding losses	(1,378)	(1,684)
Equity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, estimated fair value	10,779
Less than 12 months, gross unrealized holding losses	(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 Fund [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, estimated fair value	130	388
Less than 12 months, gross unrealized holding losses	0	(2)
12 Months or longer, estimated fair value	0	0
12 Months or longer, gross unrealized holding losses	0	0
Total, estimated fair value	130	388
Total, gross unrealized holding losses	0	(2)
OTTIs from securities		$ 300
Nonredeemable Preferred Stock [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, estimated fair value	10,649
Less than 12 months, gross unrealized holding losses	(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)
Number of securities held	0
Number of available-for-sale securities in unrealized loss positions, greater than one year	13