Other assets - Seed Capital and Private Equity Investments (Details) - Other Assets - NAV - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Fair value	$ 219	$ 201
Unfunded commitments	86	74
Seed capital
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Fair value	59	52
Unfunded commitments	25	22
Private equity investments
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Fair value	114	102
Unfunded commitments	$ 61	$ 52
Fair value, investments, entities that calculate net asset value per share, liquidating investment, remaining period	10 years	10 years
Other
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Fair value	$ 46	$ 47
Unfunded commitments	$ 0	$ 0
Minimum | Seed capital
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Fair value, investments, entities that calculate net asset value per share, liquidating investment, remaining period	3 years	3 years
Maximum | Seed capital
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Fair value, investments, entities that calculate net asset value per share, liquidating investment, remaining period	11 years	11 years
Maximum | Other
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Redemption notice period	95 days	95 days