Goodwill (Changes in the Net Carrying Amount of Goodwill by Reporting Segment) (Detail) - USD ($)		3 Months Ended	12 Months Ended
	Mar. 22, 2019	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill
Goodwill				$ 3,443,000,000
Accumulated impairment losses				(965,000,000)
Net carrying amount, beginning of period:		$ 4,252,000,000	$ 2,478,000,000
Acquisition activity		9,000,000	1,777,000,000
Sale of Business			(3,000,000)
Net carrying amount, end of period:		4,261,000,000	4,252,000,000
MB Financial, Inc.
Goodwill
Acquisition activity	$ 1,800,000,000	9,000,000
Commercial Banking
Goodwill
Goodwill				1,380,000,000
Accumulated impairment losses				(750,000,000)
Net carrying amount, beginning of period:		1,954,000,000	630,000,000
Acquisition activity		7,000,000	1,324,000,000
Sale of Business			0
Net carrying amount, end of period:		1,961,000,000	1,954,000,000
Branch Banking
Goodwill
Goodwill				1,655,000,000
Accumulated impairment losses				0
Net carrying amount, beginning of period:		2,046,000,000	1,655,000,000
Acquisition activity		1,000,000	391,000,000
Sale of Business			0
Net carrying amount, end of period:		2,047,000,000	2,046,000,000
Consumer Lending
Goodwill
Goodwill				215,000,000
Accumulated impairment losses				(215,000,000)
Net carrying amount, beginning of period:		0	0
Acquisition activity		0	0
Sale of Business			0
Net carrying amount, end of period:		0	0
Wealth and Asset Management
Goodwill
Goodwill				193,000,000
Accumulated impairment losses				0
Net carrying amount, beginning of period:		252,000,000	193,000,000
Acquisition activity		1,000,000	62,000,000
Sale of Business			(3,000,000)
Net carrying amount, end of period:		253,000,000	252,000,000
General Corporate and Other
Goodwill
Goodwill				0
Accumulated impairment losses				$ 0
Net carrying amount, beginning of period:		0	0
Acquisition activity		0	0
Sale of Business			0
Net carrying amount, end of period:		$ 0	$ 0