Supplemental Reinsurance Premiums (Schedule of Property and Liability Insurance Earned Premiums) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Abstract]
Direct amounts									$ 3,603,780	$ 3,655,233	$ 3,416,687
Ceded to other companies									(56,212)	(56,725)	(48,941)
Assumed									8,067	910	665
Net amounts	$ 921,860	$ 899,304	$ 811,897	$ 922,574	$ 925,384	$ 915,012	$ 888,776	$ 870,245	$ 3,555,635	$ 3,599,418	$ 3,368,411