Investments (Fair Values by Contractual Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair values of available-for-sale debt security investments by contractual maturity
Total available-for-sale investments	$ 31,670	$ 41,318
Total interest-bearing securities [Member]
Fair values of available-for-sale debt security investments by contractual maturity
Maturing in one year or less	9,403	6,733
Maturing after one year through three years	6,579	12,820
Maturing after three years through five years	10,934	13,836
Maturing after five years through ten years	1,788	3,263
Maturing after ten years	0	52
Mortgage- and asset-backed securities	2,966	4,465
Total available-for-sale investments	$ 31,670	$ 41,169