Available-for-sale investments (Fair Values by Contractual Maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Contractual maturity
Total available-for-sale investments	$ 37,781	$ 31,112
Total interest-bearing securities [Member]
Contractual maturity
Maturing in one year or less	8,393	4,578
Maturing after one year through three years	10,404	9,370
Maturing after three years through five years	12,157	9,932
Maturing after five years through ten years	2,974	3,087
Maturing after ten years	40	70
Mortgage- and asset-backed securities	3,659	3,939
Total available-for-sale investments	$ 37,627	$ 30,976