Investments (Maturities of Fixed Maturity Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost, Due in one year or less	$ 11,378
Amortized Cost, Due after one year through five years	62,647
Amortized Cost, Due after five years through ten years	61,043
Amortized Cost, Due after ten years	102,895
Amortized Cost, Subtotal	286,069	$ 271,676
Amortized Cost, RMBS, CMBS and ABS	48,106
Estimated Fair Value, Due in one year or less	11,437
Estimated Fair Value, Due after one year through five years	65,423
Estimated Fair Value, Due after five years through ten years	64,499
Estimated Fair Value, Due after ten years	118,254
Estimated fair value, Mortgage-backed and asset-backed securities	49,318
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $286,069 and $271,676, respectively)	$ 308,931	$ 289,563