Restrictions (Restrictions on Cash, Dividends and Other Capital Actions- Additional Information) (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Banking Subsidiary Reserve Requirement			$ 1,700	$ 1,500
Dividend Received From Nonbank Companies And Related Subsidiaries			2,000	1,900
BHC Stress Test Threshold	$ 250,000		250,000
Stock Repurchase Authorization Amount	1,240	$ 1,240	1,651	1,810	$ 1,161
Capital Distribution	$ 2,000
Change in Dividend per share	$ 0.03	$ 0.24
MB Financial, Inc.
Restricted Cash and Cash Equivalents Items [Line Items]
Dividend Received From Nonbank Companies And Related Subsidiaries			200
Yearly Average
Restricted Cash and Cash Equivalents Items [Line Items]
Banking Subsidiary Reserve Requirement			$ 1,700	$ 1,500