Equity (Components of Net Unrealized Appreciation Depreciation of Balances Carried at Fair Value) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Comprehensive Income Net Of Tax [Line Items]
Fixed maturity securities available-for-sale		$ 38,150,820	$ 32,093,625
Other invested assets		1,605,484	1,591,940
Deferred policy acquisition costs		3,239,824	3,338,605	$ 3,392,437	$ 3,342,575
Accumulated Other Comprehensive Income Loss Change In Component During Period		1,004,927	577,265	(1,028,965)
Change In Net Unrealized Appreciation Depreciation [Member]
Comprehensive Income Net Of Tax [Line Items]
Fixed maturity securities available-for-sale		987,570	561,906	(1,055,458)
Other invested assets	[1]	25,577	18,900	8,983
Effect On Unrealized Appreciation Depreciation [Member]
Comprehensive Income Net Of Tax [Line Items]
Deferred policy acquisition costs		$ (8,220)	$ (3,541)	$ 17,510

[1] Includes cash flow hedges. See Note 5 for additional information on cash flow hedges.