Investments (Amortized Cost and Fair Value of Fixed Maturity Holdings by Industry Type) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 13,413,102	$ 11,697,394
Estimated Fair Value	14,190,456	12,110,755
% of Total	100.00%	100.00%
Finance
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	4,592,728	3,838,716
Estimated Fair Value	4,827,880	3,983,623
% of Total	34.00%	32.90%
Industrial
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	7,310,121	6,607,100
Estimated Fair Value	7,740,313	6,824,063
% of Total	54.60%	56.30%
Utility
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	1,510,253	1,240,353
Estimated Fair Value	1,622,263	1,292,305
% of Total	11.40%	10.70%
Other
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost		11,225
Estimated Fair Value		$ 10,764
% of Total		0.10%