Loans and Allowance for Credit Losses - Additional Information (Details)	3 Months Ended
	Mar. 31, 2022 USD ($) loan portfolio segment	Mar. 31, 2021 USD ($)	Dec. 31, 2021 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
Number of loan portfolios | portfolio	4
Number of loan segments | segment	7
Total loans	$ 28,336,244,000		$ 13,601,846,000
Accrued interest receivable on loans	96,200,000		47,600,000
Provision for loan losses	$ 78,531,000	$ 0
Loan placed on nonaccrual when past due, number of days	90 days
Loan participations	$ 2,520,000,000
Loan participations sold	1,328,000,000
Loan participations retained	$ 1,192,000,000
TDR term	6 months
Minimum number of days for loan charge off to be recorded	120 days
Maximum number of days for loan charge off to be recorded	180 days
Financing receivable TDRs included with non-accrual loans	$ 23,800,000		11,700,000
Financing receivable TDR specific reserves	6,200,000		700,000
Unfunded commitments on TDRs	$ 0		0
Financing receivable, TDR, number of loans qualified for removal | loan	1
Charge-offs	$ (9,000)	(15,000)
Number of days for a loan to be considered to be in payment default	90 days
Troubled Debt Restructurings During Period
Financing Receivable, Allowance for Credit Losses [Line Items]
Increase in allowance for loan losses	$ 5,500,000	0
Charge-offs	0	0
First Midwest
Financing Receivable, Allowance for Credit Losses [Line Items]
Provision for loan losses	78,500,000
Increase in allowance for loan loss	96,300,000
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	8,624,253,000		3,391,769,000
Provision for loan losses	35,040,000	0
Charge-offs	0	0
Commercial | PPP Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	205,300,000		169,000,000
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 11,337,735,000		$ 6,380,674,000
Percentage of risk-based capital	236.00%
Regulatory guideline limit	300.00%
Provision for loan losses	$ 42,601,000	0
Charge-offs	$ (4,000)	$ (8,000)