Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Direct Amount									$ 2,647,488	$ 2,484,715	$ 2,330,267
Assumed From Other Companies									25,831	28,214	23,209
Ceded to Other Companies									382,292	363,357	363,567
Total Net Premiums Earned	$ 590,088	$ 572,055	$ 568,030	$ 560,854	$ 552,753	$ 542,429	$ 531,932	$ 522,458	$ 2,291,027	$ 2,149,572	$ 1,989,909
Percentage of Amount Assumed to Net									1.00%	1.00%	1.00%
Accident and Health Insurance Segment [Member]
Direct Amount									$ 24	$ 32	$ 37
Assumed From Other Companies									0	0	0
Ceded to Other Companies									24	0	37
Total Net Premiums Earned									$ 0	$ 32	$ 0
Percentage of Amount Assumed to Net									0.00%	0.00%	0.00%
Property and liability insurance [Member]
Direct Amount									$ 2,647,464	$ 2,484,683	$ 2,330,230
Assumed From Other Companies									25,831	28,214	23,209
Ceded to Other Companies									382,268	363,357	363,530
Total Net Premiums Earned									$ 2,291,027	$ 2,149,540	$ 1,989,909
Percentage of Amount Assumed to Net									1.00%	1.00%	1.00%