Investments (Amortized Cost and Fair Value of Fixed Maturity Holdings by Industry Type) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 18,762,648	$ 17,575,507
Estimated Fair Value	$ 20,164,848	$ 17,708,156
% of Total	100.00%	100.00%
Financial Services Sector [Member]
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 6,530,758	$ 5,408,791
Estimated Fair Value	$ 6,916,195	$ 5,555,044
% of Total	34.40%	31.40%
Commercial and Industrial Sector [Member]
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 10,241,489	$ 10,211,426
Estimated Fair Value	$ 11,028,232	$ 10,129,917
% of Total	54.60%	57.20%
Utility Sector [Member]
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 1,990,401	$ 1,955,290
Estimated Fair Value	$ 2,220,421	$ 2,023,195
% of Total	11.00%	11.40%