Investments (Amortized Cost and Fair Value of Fixed Maturity Holdings by Industry Type) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 16,000,887	$ 14,010,604
Estimated Fair Value	$ 16,489,406	$ 14,885,583
% of Total	100.00%	100.00%
Finance
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 5,238,400	$ 4,789,568
Estimated Fair Value	$ 5,419,421	$ 5,066,408
% of Total	32.90%	34.00%
Industrial
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 9,057,282	$ 7,639,330
Estimated Fair Value	$ 9,281,262	$ 8,086,067
% of Total	56.20%	54.30%
Utility
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 1,705,205	$ 1,581,706
Estimated Fair Value	$ 1,788,723	$ 1,733,108
% of Total	10.90%	11.70%