Goodwill and intangible assets - Intangible Assets by Type (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Intangible Assets by Major Class [Line Items]
Gross carrying amount	$ 4,303	$ 4,790
Accumulated amortization	(1,362)	(1,778)
Net carrying amount	2,941	3,012	$ 3,026	$ 3,107
Finite-lived Intangible Assets
Intangible Assets by Major Class [Line Items]
Gross carrying amount	1,714	2,199
Accumulated amortization	(1,362)	(1,778)
Net carrying amount	$ 352	421
Finite-lived Intangible Assets | Weighted Average
Intangible Assets by Major Class [Line Items]
Remaining weighted- average amortization period	10 years
Finite-lived Intangible Assets | Customer contracts—Investment Services
Intangible Assets by Major Class [Line Items]
Gross carrying amount	$ 1,059	1,435
Accumulated amortization	(864)	(1,199)
Net carrying amount	$ 195	236
Finite-lived Intangible Assets | Customer contracts—Investment Services | Weighted Average
Intangible Assets by Major Class [Line Items]
Remaining weighted- average amortization period	9 years
Finite-lived Intangible Assets | Customer relationships
Intangible Assets by Major Class [Line Items]
Gross carrying amount	$ 609	705
Accumulated amortization	(491)	(564)
Net carrying amount	$ 118	141
Finite-lived Intangible Assets | Customer relationships | Weighted Average
Intangible Assets by Major Class [Line Items]
Remaining weighted- average amortization period	9 years
Finite-lived Intangible Assets | Other
Intangible Assets by Major Class [Line Items]
Gross carrying amount	$ 46	59
Accumulated amortization	(7)	(15)
Net carrying amount	$ 39	44
Finite-lived Intangible Assets | Other | Weighted Average
Intangible Assets by Major Class [Line Items]
Remaining weighted- average amortization period	14 years
Indefinite-lived Intangible Assets
Intangible Assets by Major Class [Line Items]
Gross carrying amount	$ 2,589	2,591
Net carrying amount	2,589	2,591
Indefinite-lived Intangible Assets | Tradenames
Intangible Assets by Major Class [Line Items]
Gross carrying amount	1,294	1,295
Net carrying amount	1,294	1,295
Indefinite-lived Intangible Assets | Customer relationships
Intangible Assets by Major Class [Line Items]
Gross carrying amount	1,295	1,296
Net carrying amount	$ 1,295	$ 1,296