Investments (Amortized Cost and Fair Value of Fixed Maturity Holdings by Industry Type) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 26,055,078	$ 24,006,407
Estimated Fair Value	$ 27,635,431	$ 23,982,119
% of Total	100.00%	100.00%
Financial Services Sector [Member]
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 9,865,739	$ 8,793,742
Estimated Fair Value	$ 10,385,443	$ 8,730,568
% of Total	37.60%	36.30%
Commercial and Industrial Sector [Member]
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 13,063,121	$ 12,336,857
Estimated Fair Value	$ 13,878,056	$ 12,342,111
% of Total	50.20%	51.60%
Utility Sector [Member]
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 3,126,218	$ 2,875,808
Estimated Fair Value	$ 3,371,932	$ 2,909,440
% of Total	12.20%	12.10%