Acquisition Activity - Total Consideration (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended
	Feb. 15, 2022	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021	Dec. 31, 2020
Assets
Goodwill		$ 1,997,157	$ 1,036,994	$ 1,036,994	$ 1,036,994
Fair value of consideration
Issuance of common stock		2,446,312
Preferred Stock
Fair value of consideration
Issuance of common stock		243,870	0
Common Stock
Fair value of consideration
Issuance of common stock	$ 129,365	$ 129,365	$ 0
Share price (in dollars per share)	$ 18.92
First Midwest
Assets
Cash and cash equivalents	$ 1,912,629
Investment securities	3,526,278
FHLB/Federal Reserve Bank stock	106,097
Loans held for sale	13,809
Loans, net of allowance for credit losses	14,308,309
Premises and equipment	108,306
Operating lease right-of-use assets	134,388
Accrued interest receivable	53,502
Goodwill	960,163
Other intangible assets	117,584
Company-owned life insurance	301,025
Other assets	322,267
Total assets	21,864,357
Liabilities
Deposits	17,249,404
Securities sold under agreements to repurchase	135,194
Federal Home Loan Bank advances	1,158,623
Other borrowings	274,569
Accrued expenses and other liabilities	356,385
Total liabilities	19,174,175
Fair value of consideration
Total consideration	$ 2,690,182
First Midwest | Preferred Stock
Fair value of consideration
Issuance of common stock shares for acquisitions of business	243,870
First Midwest | Common Stock
Fair value of consideration
Issuance of common stock shares for acquisitions of business	2,446,312