Available-for-sale investments (Fair Values by Contractual Maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Contractual maturity
Total available-for-sale investments	$ 41,318	$ 37,781
Total interest-bearing securities [Member]
Contractual maturity
Maturing in one year or less	6,733	8,393
Maturing after one year through three years	12,820	10,404
Maturing after three years through five years	13,836	12,157
Maturing after five years through ten years	3,263	2,974
Maturing after ten years	52	40
Mortgage- and asset-backed securities	4,465	3,659
Total available-for-sale investments	$ 41,169	$ 37,627