Commitments and contingent liabilities - Significant Industry Concentrations Related to Credit Exposure (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Loans	$ 55,491	$ 54,953
Financial institutions
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Loans	11,000
Unfunded commitments	33,500
Total exposure	44,500
Financial institutions | Securities industry
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Loans	2,800
Unfunded commitments	22,300
Total exposure	25,100
Financial institutions | Banks
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Loans	6,100
Unfunded commitments	1,100
Total exposure	7,200
Financial institutions | Asset managers
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Loans	1,200
Unfunded commitments	6,500
Total exposure	7,700
Financial institutions | Insurance
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Loans	100
Unfunded commitments	2,700
Total exposure	2,800
Financial institutions | Government
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Loans	100
Unfunded commitments	200
Total exposure	300
Financial institutions | Other
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Loans	700
Unfunded commitments	700
Total exposure	1,400
Commercial
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Loans	1,900
Unfunded commitments	12,200
Total exposure	14,100
Commercial | Services and other
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Loans	1,000
Unfunded commitments	3,500
Total exposure	4,500
Commercial | Manufacturing
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Loans	700
Unfunded commitments	3,800
Total exposure	4,500
Commercial | Energy and utilities
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Loans	200
Unfunded commitments	4,000
Total exposure	4,200
Commercial | Media and telecom
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Loans	0
Unfunded commitments	900
Total exposure	$ 900