Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	$ 6,973,216	$ 6,726,029	$ 6,647,600
Ceded to Other Companies	1,269,267	1,216,455	1,119,788
Assumed from Other Companies	729,278	750,934	896,101
Total net premiums written	$ 6,433,227	$ 6,260,508	$ 6,423,913
Percentage of Amount Assumed to Net	11.30%	12.00%	13.90%
Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	$ 6,959,830	$ 6,707,916	$ 6,634,540
Ceded to Other Companies	1,204,509	1,153,960	1,051,887
Assumed from Other Companies	197,540	161,915	160,967
Total net premiums written	$ 5,952,861	$ 5,715,871	$ 5,743,620
Percentage of Amount Assumed to Net	3.30%	2.80%	2.80%
Reinsurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	$ 13,386	$ 18,113	$ 13,060
Ceded to Other Companies	64,758	62,495	67,901
Assumed from Other Companies	531,738	589,019	735,134
Total net premiums written	$ 480,366	$ 544,637	$ 680,293
Percentage of Amount Assumed to Net	110.70%	108.10%	108.10%