Investments (Amortized Cost and Fair Value of Fixed Maturity Holdings by Industry Type) (Details) - Corporate Debt Securities [Member] - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 28,154,301	$ 24,006,407
Estimated Fair Value	$ 30,353,219	$ 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	$ 10,604,290	$ 8,793,742
Estimated Fair Value	$ 11,327,324	$ 8,730,568
% of Total	37.30%	36.30%
Commercial and Industrial Sector [Member]
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 14,099,496	$ 12,336,857
Estimated Fair Value	$ 15,213,256	$ 12,342,111
% of Total	50.10%	51.60%
Utility Sector [Member]
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 3,450,515	$ 2,875,808
Estimated Fair Value	$ 3,812,639	$ 2,909,440
% of Total	12.60%	12.10%