Investments (Amortized Cost and Fair Value of Fixed Maturity Holdings by Industry Type) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 21,418,741	$ 18,924,711
Estimated Fair Value	$ 22,535,830	$ 19,619,084
% of Total	100.00%	100.00%
Financial Services Sector [Member]
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 7,797,576	$ 6,725,199
Estimated Fair Value	$ 8,146,891	$ 6,888,968
% of Total	36.20%	35.20%
Commercial and Industrial Sector [Member]
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 11,323,024	$ 10,228,813
Estimated Fair Value	$ 11,914,845	$ 10,639,613
% of Total	52.80%	54.20%
Utility Sector [Member]
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 2,298,141	$ 1,970,699
Estimated Fair Value	$ 2,474,094	$ 2,090,503
% of Total	11.00%	10.60%