Investments (Amortized Cost and Fair Value of Fixed Maturitiy Holdings by Industry Type) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 11,858,578	$ 11,697,394
Estimated Fair Value	12,518,300	12,110,755
% of Total	100.00%	100.00%
Finance
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	3,993,669	3,838,716
Estimated Fair Value	4,204,783	3,983,623
% of Total	33.60%	32.90%
Industrial
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	5,967,291	5,943,353
Estimated Fair Value	6,295,510	6,138,150
% of Total	50.30%	50.70%
Utility
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	1,886,833	1,904,100
Estimated Fair Value	2,007,246	1,978,218
% of Total	16.00%	16.30%
Other
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	10,785	11,225
Estimated Fair Value	$ 10,761	$ 10,764
% of Total	0.10%	0.10%