Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Real estate — commercial mortgage
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	$ 565	$ 509	$ 539	$ 502
Servicing retained from loan sales	30	30	107	71
Purchases	6	10	24	33
Amortization	(29)	(29)	(88)	(86)
Temporary impairments	(6)	(2)	(16)	(2)
Balance at end of period	566	518	566	518
Fair value at end of period	685	681	685	681
Agency residential mortgage-backed securities
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	46	39	46	37
Servicing retained from loan sales	12	5	25	9
Purchases	0	0	0	0
Amortization	(4)	(3)	(9)	(5)
Temporary impairments	(1)	0	(9)	0
Balance at end of period	53	41	53	41
Fair value at end of period	$ 54	$ 43	$ 54	$ 43