Reinsurance (Effect of Reinsurance on Net Premiums and Claims and Other Policy Benefits) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums Earned, Net [Abstract]
Direct Premiums Earned	$ 61,571	$ 57,562	$ 43,106
Assumed Premiums Earned	10,642,462	10,049,587	9,371,308
Ceded Premiums Earned	(862,903)	(858,278)	(843,673)
Net premiums	9,841,130	9,248,871	8,570,741
Policyholder Benefits and Claims Incurred, Net [Abstract]
Policyholder Benefits and Claims Incurred, Direct	104,447	105,435	82,942
Policyholder Benefits and Claims Incurred, Assumed	9,281,590	8,621,647	8,205,308
Policyholder Benefits and Claims Incurred, Ceded	(867,120)	(733,707)	(798,868)
Claims and other policy benefits	$ 8,518,917	$ 7,993,375	$ 7,489,382