Available-for-sale investments (Fair Values by Contractual Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Fair values of available-for-sale debt security investments by contractual maturity
Maturing in one year or less	$ 3,965	$ 4,578
Maturing after one year through three years	11,141	9,370
Maturing after three years through five years	12,155	9,932
Maturing after five years through ten years	3,335	3,087
Maturing after ten years	53	70
Mortgage- and asset-backed securities	3,918	3,939
Total available-for-sale investments	34,690	31,112
Total interest-bearing securities [Member]
Fair values of available-for-sale debt security investments by contractual maturity
Total available-for-sale investments	$ 34,567	$ 30,976