Reinsurance (Reinsurance Financial Information) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Disclosures [Abstract]
Due from reinsurers	$ 1,783,200	$ 1,743,980
Written premiums, Direct	6,726,029	6,647,600	$ 6,412,533
Written premiums, Assumed	750,934	896,101	837,460
Written premiums, Ceded	(1,216,455)	(1,119,788)	(1,060,478)
Total net premiums written	6,260,508	6,423,913	6,189,515
Earned premiums, Direct	6,661,046	6,492,240	6,245,714
Earned premiums, Assumed	812,309	900,570	845,735
Earned premiums, Ceded	(1,161,936)	(1,099,462)	(1,050,840)
Net premiums earned	6,311,419	6,293,348	6,040,609
Ceded losses incurred	601,769	707,336	501,999
Uncollectible reinsurance	1,010	1,049	1,020
Fees and Commissions, Other	$ 241,983	$ 201,957	$ 173,288