Investments (Amortized Cost and Fair Value of Fixed Maturity Holdings by Industry Type) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 18,924,711	$ 17,575,507
Available-for-sale Securities	$ 19,619,084	$ 17,708,156
Percentage Of Total Available For Sales Securities	100.00%	100.00%
Financial Services Sector [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 6,725,199	$ 5,408,791
Available-for-sale Securities	$ 6,888,968	$ 5,555,044
Percentage Of Total Available For Sales Securities	35.20%	31.40%
Commercial and Industrial Sector [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 10,228,813	$ 10,211,426
Available-for-sale Securities	$ 10,639,613	$ 10,129,917
Percentage Of Total Available For Sales Securities	54.20%	57.20%
Utility Industry [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 1,970,699	$ 1,955,290
Available-for-sale Securities	$ 2,090,503	$ 2,023,195
Percentage Of Total Available For Sales Securities	10.60%	11.40%