Investments (Amortized Cost and Fair Value of Fixed Maturity Holdings by Industry Type) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 24,216,876	$ 24,006,407
Estimated Fair Value	$ 25,077,338	$ 23,982,119
% of Total	100.00%	100.00%
Finance
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 8,981,541	$ 8,793,742
Estimated Fair Value	$ 9,249,273	$ 8,730,568
% of Total	36.90%	36.30%
Industrial
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 12,316,131	$ 12,336,857
Estimated Fair Value	$ 12,759,526	$ 12,342,111
% of Total	50.90%	51.60%
Utility
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 2,919,204	$ 2,875,808
Estimated Fair Value	$ 3,068,539	$ 2,909,440
% of Total	12.20%	12.10%