Available-for-sale securities - Summary of available-for-sale debt securities by contractual maturity (Details) $ in Millions	Dec. 31, 2017 USD ($)
Investments, Debt and Equity Securities [Abstract]
Within one year, amortized cost	$ 23,139
After one year through five years, amortized cost	11,125
After five years through ten years, amortized cost	98
After ten years, amortized cost	37
Debt securities, total amortized cost	34,399
Within one year, fair value	23,117
After one year through five years, fair value	11,051
After five years through ten years, fair value	96
After ten years, fair value	37
Debt Securities, Fair Value	$ 34,301