Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	$ 6,647,600	$ 6,412,533	$ 6,185,242
Ceded to Other Companies	1,119,788	1,060,478	1,065,891
Assumed from Other Companies	896,101	837,460	877,596
Total net premiums written	$ 6,423,913	$ 6,189,515	$ 5,996,947
Percentage of Amount Assumed to Net	13.90%	13.50%	14.60%
Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	$ 6,634,540	$ 6,395,806	$ 6,142,648
Ceded to Other Companies	1,059,149	1,016,095	1,022,287
Assumed from Other Companies	200,522	211,686	225,302
Total net premiums written	$ 5,775,913	$ 5,591,397	$ 5,345,663
Percentage of Amount Assumed to Net	3.50%	3.80%	4.20%
Reinsurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	$ 13,060	$ 16,727	$ 42,594
Ceded to Other Companies	60,639	44,383	43,604
Assumed from Other Companies	695,579	625,774	652,294
Total net premiums written	$ 648,000	$ 598,118	$ 651,284
Percentage of Amount Assumed to Net	107.30%	104.60%	100.20%