Investments (Amortized Cost and Fair Value of Fixed Maturity Holdings by Industry Type) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 14,010,604	$ 11,697,394
Available-for-sale Securities	14,885,583	12,110,755
Percentage Of Total Available For Sales Securities	100.00%	100.00%
Finance Industry [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	4,789,568	3,838,716
Available-for-sale Securities	5,066,408	3,983,623
Percentage Of Total Available For Sales Securities	34.00%	32.90%
Industrial [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	7,639,330	6,607,100
Available-for-sale Securities	8,086,067	6,824,063
Percentage Of Total Available For Sales Securities	54.30%	56.30%
Utility Industry [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,581,706	1,240,353
Available-for-sale Securities	1,733,108	1,292,305
Percentage Of Total Available For Sales Securities	11.70%	10.70%
Other Industry [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis		11,225
Available-for-sale Securities		$ 10,764
Percentage Of Total Available For Sales Securities		0.10%