Restrictions on Cash, Dividends and Other Capital Actions (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Mar. 26, 2020
Restricted Cash and Cash Equivalents Items [Line Items]
Reserve requirement		$ 0	$ 1,700,000,000		$ 0
Dividends from consolidated nonbank subsidiaries		$ 1,300,000,000	$ 2,000,000,000.0
Common stock, per share (in usd per share)	$ 0.27	$ 1.08	$ 0.94	$ 0.74
MB Financial, Inc.
Restricted Cash and Cash Equivalents Items [Line Items]
Dividends from consolidated nonbank subsidiaries			$ 200,000,000
Yearly Average
Restricted Cash and Cash Equivalents Items [Line Items]
Reserve requirement			$ 1,700,000,000