Goodwill (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 22, 2019	Mar. 31, 2020	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill
Goodwill					$ 3,443
Accumulated impairment losses					(965)
Goodwill [Roll Forward]
Net carrying value, beginning of period		$ 4,252	$ 4,252	$ 2,478
Acquisition activity			9	1,777
Sale of business				(3)
Net carrying value, end of period			4,261	4,252
MB Financial, Inc.
Goodwill [Roll Forward]
Acquisition activity	$ 1,800	9
Net carrying value, end of period	$ 1,786
Commercial Banking
Goodwill
Goodwill					1,380
Accumulated impairment losses					(750)
Goodwill [Roll Forward]
Net carrying value, beginning of period		1,954	1,954	630
Acquisition activity			7	1,324
Sale of business				0
Net carrying value, end of period			1,961	1,954
Branch Banking
Goodwill
Goodwill					1,655
Accumulated impairment losses					0
Goodwill [Roll Forward]
Net carrying value, beginning of period		2,046	2,046	1,655
Acquisition activity			1	391
Sale of business				0
Net carrying value, end of period			2,047	2,046
Consumer Lending
Goodwill
Goodwill					215
Accumulated impairment losses					(215)
Goodwill [Roll Forward]
Net carrying value, beginning of period		0	0	0
Acquisition activity			0	0
Sale of business				0
Net carrying value, end of period			0	0
Wealth and Asset Management
Goodwill
Goodwill					193
Accumulated impairment losses					0
Goodwill [Roll Forward]
Net carrying value, beginning of period		252	252	193
Acquisition activity			1	62
Sale of business				(3)
Net carrying value, end of period			253	252
General Corporate and Other
Goodwill
Goodwill					0
Accumulated impairment losses					$ 0
Goodwill [Roll Forward]
Net carrying value, beginning of period		$ 0	0	0
Acquisition activity			0	0
Sale of business				0
Net carrying value, end of period			$ 0	$ 0