Investments (Amortized Cost and Fair Value of Fixed Maturity Holdings by Industry Type) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 17,575,507	$ 14,010,604
Available-for-sale Securities	$ 17,708,156	$ 14,885,583
Percentage Of Total Available For Sales Securities	100.00%	100.00%
Financial Services Sector [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 5,408,791	$ 4,789,568
Available-for-sale Securities	$ 5,555,044	$ 5,066,408
Percentage Of Total Available For Sales Securities	31.40%	34.00%
Commercial and Industrial Sector [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 10,211,426	$ 7,639,330
Available-for-sale Securities	$ 10,129,917	$ 8,086,067
Percentage Of Total Available For Sales Securities	57.20%	54.30%
Utility Industry [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 1,955,290	$ 1,581,706
Available-for-sale Securities	$ 2,023,195	$ 1,733,108
Percentage Of Total Available For Sales Securities	11.40%	11.70%