Available-for-sale investments (Fair Values by Contractual Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fair values of available-for-sale debt security investments by contractual maturity
Maturing in one year or less	$ 3,859	$ 4,936
Maturing after one year through three years	9,331	6,829
Maturing after three years through five years	10,093	7,840
Maturing after five years through ten years	3,433	3,267
Maturing after ten years	216	218
Mortgage- and asset-backed securities	3,733	3,498
Total available-for-sale investments	30,800	26,732
Total interest-bearing securities [Member]
Fair values of available-for-sale debt security investments by contractual maturity
Total available-for-sale investments	$ 30,665	$ 26,588