Supplemental Reinsurance Premiums (Schedule of Property and Liability Insurance Earned Premiums) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information [Abstract]
Direct amounts									$ 3,221,493	$ 3,146,864	$ 2,970,424
Ceded to other companies									(26,881)	(15,846)	(12,964)
Assumed									825	755	437
Net amounts	$ 806,796	$ 801,205	$ 797,666	$ 789,770	$ 794,517	$ 790,850	$ 779,321	$ 767,085	$ 3,195,437	$ 3,131,773	$ 2,957,897