Other assets - Seed Capital and Private Equity Investments (Details) - Other Assets - NAV - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Fair value	$ 158	$ 181
Unfunded commitments	52	55
Seed capital
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Fair value	51	59
Unfunded commitments	0	0
Private equitiy investments (SBICs)
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Fair value	80	89
Unfunded commitments	52	55
Other
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Fair value	27	33
Unfunded commitments	$ 0	$ 0
Maximum | Other
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Redemption notice period	95 days	95 days