Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in stockholders' equity attributable to parent		$ 5,150.0	$ 4,959.0	$ 4,883.0	$ 4,608.0
Financing receivable, allowance for credit loss, period increase (decrease)	$ 105.0
Allowance for credit loss		$ 344.0	363.0
Initial ACL on PCD Loans	50.3
Allowances for impaired commercial loans				1.0
Core deposit intangibles amortization period, years		10 years
Largest amount recognized in the financial statement of tax benefit threshold, minimum		50.00%
Retained Earnings
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in stockholders' equity attributable to parent		$ 1,110.0	$ 869.0	798.0	$ 576.0
Accounting Standards Update 2016-13 | Longer duration loans and gross up
Summary Of Significant Accounting Policies [Line Items]
Financing receivable, allowance for credit loss, period increase (decrease)	50.0
Accounting Standards Update 2016-13 | Unfunded loan commitment
Summary Of Significant Accounting Policies [Line Items]
Allowance for credit loss	10.0
Impact of ASC 326 Adoption
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in stockholders' equity attributable to parent				(50.0)
Impact of ASC 326 Adoption | Retained Earnings
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in stockholders' equity attributable to parent				$ (50.0)
Impact of ASC 326 Adoption | Accounting Standards Update 2016-13 | Retained Earnings
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in stockholders' equity attributable to parent	$ (50.6)
Restricted Stock Units (RSUs)
Summary Of Significant Accounting Policies [Line Items]
Vesting period of awards issued, years		3 years
Commercial Loan
Summary Of Significant Accounting Policies [Line Items]
Threshold period past due for non-accrual loans		90 days
Installment Loans
Summary Of Significant Accounting Policies [Line Items]
Threshold period past due for non-accrual loans		120 days
Residential Mortgages and Consumer Lines of Credit
Summary Of Significant Accounting Policies [Line Items]
Threshold period past due for non-accrual loans		180 days
Minimum
Summary Of Significant Accounting Policies [Line Items]
Useful life of the asset, years		3 years
Estimated useful life of intangibles, years		8 years
Minimum | Commercial Loan
Summary Of Significant Accounting Policies [Line Items]
Financing receivable, individual reserves in default, threshold		$ 1.0
Maximum
Summary Of Significant Accounting Policies [Line Items]
Useful life of the asset, years		39 years
Estimated useful life of intangibles, years		13 years
Maximum | Commercial and Consumer Loans
Summary Of Significant Accounting Policies [Line Items]
Financing receivable, individual reserves in default, threshold		$ 1.0