Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	$ 6,726,029	$ 6,647,600	$ 6,412,533
Ceded to Other Companies	1,216,455	1,119,788	1,060,478
Assumed from Other Companies	750,934	896,101	837,460
Total net premiums written	$ 6,260,508	$ 6,423,913	$ 6,189,515
Percentage of Amount Assumed to Net	12.00%	13.90%	13.50%
Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	$ 6,707,916	$ 6,634,540	$ 6,395,806
Ceded to Other Companies	1,153,960	1,051,887	1,009,711
Assumed from Other Companies	161,915	160,967	169,342
Total net premiums written	$ 5,715,871	$ 5,743,620	$ 5,555,437
Percentage of Amount Assumed to Net	2.80%	2.80%	3.00%
Reinsurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	$ 18,113	$ 13,060	$ 16,727
Ceded to Other Companies	62,495	67,901	50,767
Assumed from Other Companies	589,019	735,134	668,118
Total net premiums written	$ 544,637	$ 680,293	$ 634,078
Percentage of Amount Assumed to Net	108.10%	108.10%	105.40%