Schedule IV. Reinsurance (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 5,314,548	$ 5,491,932	$ 5,366,061
Ceded to Other Companies	1,023,197	1,165,868	1,130,059
Assumed From Other Companies	0	0	0
Net Amount	4,291,351	4,326,064	4,236,002
Total [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	1,067,128	972,413	864,842
Ceded to Other Companies	60,764	59,919	52,777
Assumed From Other Companies	17,037	18,396	16,265
Net Amount	$ 1,023,401	$ 930,890	$ 828,330
Percentage of Amount Assumed to Net Earned	1.66%	1.98%	1.96%
Property and Casualty Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 977,090	$ 890,057	$ 800,492
Ceded to Other Companies	57,996	56,758	49,818
Assumed From Other Companies	17,037	18,396	16,265
Net Amount	$ 936,131	$ 851,695	$ 766,939
Percentage of Amount Assumed to Net Earned	1.82%	2.16%	2.12%
Life Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 90,038	$ 82,356	$ 64,350
Ceded to Other Companies	2,768	3,161	2,959
Assumed From Other Companies	0	0	0
Net Amount	$ 87,270	$ 79,195	$ 61,391
Percentage of Amount Assumed to Net Earned	0.00%	0.00%	0.00%