Other assets - Seed Capital and Private Equity Investments (Details) - Other Assets - NAV - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Fair value	$ 187	$ 181
Unfunded commitments	70	55
Seed capital
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Fair value	56	59
Unfunded commitments	12	0
Private equitiy investments (SBICs)
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Fair value	91	89
Unfunded commitments	58	55
Other
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Fair value	40	33
Unfunded commitments	$ 0	$ 0
Minimum | Seed capital
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Redemption notice period	6 years	6 years
Maximum | Seed capital
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Redemption notice period	11 years
Maximum | Other
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Redemption notice period	95 days	95 days