Acquisition and Divestiture Activity - Total Consideration (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended	9 Months Ended
	Feb. 15, 2022	Mar. 31, 2022	Sep. 30, 2022	Sep. 30, 2021	Jun. 30, 2022	Dec. 31, 2021	Jun. 30, 2021	Dec. 31, 2020
Assets
Goodwill			$ 2,002,599	$ 1,036,994	$ 1,991,534	$ 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	$ 2,446,312	$ 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	14,298,873
Premises and equipment	111,867
Operating lease right-of-use assets	129,698
Accrued interest receivable	53,502
Goodwill	965,605
Other intangible assets	117,584
Company-owned life insurance	301,025
Other assets	315,241
Total assets	21,852,208
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	344,236
Total liabilities	19,162,026
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 (in shares)	243,870
First Midwest | Common Stock
Fair value of consideration
Issuance of common stock shares for acquisitions of business (in shares)	2,446,312