Investments (Amortized Cost and Fair Value of Fixed Maturity Holdings by Industry Type) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 21,966,803	$ 18,924,711
Available-for-sale Securities	$ 23,210,968	$ 19,619,084
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	$ 7,977,885	$ 6,725,199
Available-for-sale Securities	$ 8,362,774	$ 6,888,968
Percentage Of Total Available For Sales Securities	36.10%	35.20%
Commercial and Industrial Sector [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 11,535,166	$ 10,228,813
Available-for-sale Securities	$ 12,199,333	$ 10,639,613
Percentage Of Total Available For Sales Securities	52.50%	54.20%
Utility Industry [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 2,453,752	$ 1,970,699
Available-for-sale Securities	$ 2,648,861	$ 2,090,503
Percentage Of Total Available For Sales Securities	11.40%	10.60%