Schedule IV. Reinsurance (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 5,309,508	$ 5,314,548	$ 5,491,932
Ceded to Other Companies	1,014,794	1,023,197	1,165,868
Assumed From Other Companies	0	0	0
Net Amount	4,294,714	4,291,351	4,326,064
Total [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	1,107,828	1,067,128	972,413
Ceded to Other Companies	64,027	60,764	59,919
Assumed From Other Companies	15,059	17,037	18,396
Net Amount	$ 1,058,860	$ 1,023,401	$ 930,890
Percentage of Amount Assumed to Net Earned	1.42%	1.66%	1.98%
Continuing Operations - Property and Casualty Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 1,043,738	$ 977,090	$ 890,057
Ceded to Other Companies	61,305	57,996	56,758
Assumed From Other Companies	15,059	17,037	18,396
Net Amount	$ 997,492	$ 936,131	$ 851,695
Percentage of Amount Assumed to Net Earned	1.51%	1.82%	2.16%
Discontinued Operations | Discontinued Operations - Life, accident and health insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 64,090	$ 90,038	$ 82,356
Ceded to Other Companies	2,722	2,768	3,161
Assumed From Other Companies	0	0	0
Net Amount	$ 61,368	$ 87,270	$ 79,195
Percentage of Amount Assumed to Net Earned	0.00%	0.00%	0.00%