Investments (Amortized Cost and Fair Value of Fixed Maturitiy Holdings by Industry Type) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 13,280,721	$ 11,697,394
Estimated Fair Value	14,151,411	12,110,755
% of Total	100.00%	100.00%
Finance
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	4,512,831	3,838,716
Estimated Fair Value	4,788,259	3,983,623
% of Total	33.80%	32.90%
Industrial
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	7,240,086	6,607,100
Estimated Fair Value	7,729,309	6,824,063
% of Total	54.60%	56.30%
Utility
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	1,516,641	1,240,353
Estimated Fair Value	1,622,357	1,292,305
% of Total	11.50%	10.70%
Other
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	11,163	11,225
Estimated Fair Value	$ 11,486	$ 10,764
% of Total	0.10%	0.10%