Investments (Amortized Cost and Fair Value of Fixed Maturitiy Holdings by Industry Type) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 11,505,673	$ 11,333,431
Estimated Fair Value	11,934,562	12,380,071
% of Total	100.00%	100.00%
Finance
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	3,728,120	3,619,455
Estimated Fair Value	3,870,583	3,900,152
% of Total	32.40%	31.50%
Industrial
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	5,926,283	5,881,967
Estimated Fair Value	6,125,796	6,443,846
% of Total	51.30%	52.00%
Utility
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	1,829,273	1,799,658
Estimated Fair Value	1,916,477	2,002,611
% of Total	16.10%	16.20%
Other
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	21,997	32,351
Estimated Fair Value	$ 21,706	$ 33,462
% of Total	0.20%	0.30%