Reinsurance (Reinsurance Financial Information) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reinsurance Disclosures [Abstract]
Due from reinsurers	$ 1,743,980	$ 1,532,829
Written premiums, Direct	6,647,600	6,412,533	$ 6,185,242
Written premiums, Assumed	896,101	837,460	877,596
Written premiums, Ceded	(1,119,788)	(1,060,478)	(1,065,891)
Total net premiums written	6,423,913	6,189,515	5,996,947
Earned premiums, Direct	6,492,240	6,245,714	5,889,021
Earned premiums, Assumed	900,570	845,735	886,063
Earned premiums, Ceded	(1,099,462)	(1,050,840)	(1,030,666)
Net premiums earned	6,293,348	6,040,609	5,744,418
Ceded losses incurred	707,336	501,999	475,802
Uncollectible reinsurance	1,049	1,020	1,144
Fees and Commissions, Other	$ 201,957	$ 173,288	$ 160,215