Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Commercial Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	$ 509	$ 451	$ 502	$ 412
Servicing retained from loan sales	30	25	71	81
Purchases	10	17	33	50
Amortization	(29)	(25)	(86)	(75)
Temporary impairments	(2)	0	(2)	0
Balance at end of period	518	468	518	468
Fair value at end of period	681	672	681	672
Residential Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	39	34	37	31
Servicing retained from loan sales	5	2	9	7
Purchases	0	0	0	0
Amortization	(3)	(1)	(5)	(3)
Balance at end of period	41	35	41	35
Fair value at end of period	$ 43	$ 49	$ 43	$ 49