Investments (Amortized Cost and Fair Value of Fixed Maturity Holdings by Industry Type) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 19,281,539	$ 18,924,711
Estimated Fair Value	$ 20,161,453	$ 19,619,084
% of Total	100.00%	100.00%
Finance
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 6,862,721	$ 6,725,199
Estimated Fair Value	$ 7,103,865	$ 6,888,968
% of Total	35.20%	35.20%
Industrial
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 10,398,548	$ 10,228,813
Estimated Fair Value	$ 10,889,185	$ 10,639,613
% of Total	54.00%	54.20%
Utility
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 2,020,270	$ 1,970,699
Estimated Fair Value	$ 2,168,403	$ 2,090,503
% of Total	10.80%	10.60%