Investments (Maturities of Fixed Maturity Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost, Due in one year or less	$ 12,620
Amortized Cost, Due after one year through five years	60,639
Amortized Cost, Due after five years through ten years	61,373
Amortized Cost, Due after ten years	106,222
Amortized Cost, Structured Securities	46,245
Amortized Cost, Subtotal	287,099	$ 286,069
Estimated Fair Value, Due in one year or less	12,674
Estimated Fair Value, Due after one year through five years	62,809
Estimated Fair Value, Due after five years through ten years	63,919
Estimated Fair Value, Due after ten years	118,424
Estimated Fair Value, Structured Securities	46,885
Available-for-sale Securities, Debt Securities	$ 304,711	$ 308,931