Supplemental Reinsurance Premiums (Schedule of Property and Liability Insurance Earned Premiums) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplementary Insurance Information [Abstract]
Direct amounts									$ 3,146,864	$ 2,970,424	$ 2,806,889
Ceded to other companies									(15,846)	(12,964)	(11,185)
Assumed									755	437	491
Net amounts	$ 794,517	$ 790,850	$ 779,321	$ 767,085	$ 760,094	$ 745,520	$ 731,546	$ 720,737	$ 3,131,773	$ 2,957,897	$ 2,796,195