Investments - Contractual maturity (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Due in one year or less, cost or amortized cost	$ 1,374	$ 1,779
Due after one year through five years, cost or amortized cost	7,931	7,566
Due after five years through ten years, cost or amortized cost	15,853	15,892
Due after ten years, cost or amortized cost	13,652	13,112
Total Amortized Cost Basis	38,810	38,349
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Due in one year or less, estimated fair value	1,404	1,828
Due after one year through five years, estimated fair value	8,293	7,955
Due after five years through ten years, estimated fair value	16,574	16,332
Due after ten years, estimated fair value	15,815	14,778
Total Estimated Fair Value	$ 42,086	$ 40,893