Fair value measurement - Assets on Nonrecurring Basis (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Decrease in fair value of loans of underlying collateral (less than)	$ 1	$ 1
Measured at fair value on a nonrecurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	52	58
Other assets	113	64
Total assets	165	122
Measured at fair value on a nonrecurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0
Other assets	0	0
Total assets	0	0
Measured at fair value on a nonrecurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	52	58
Other assets	113	64
Total assets	165	122
Measured at fair value on a nonrecurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0
Other assets	0	0
Total assets	$ 0	$ 0