Loans and Leases - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) State	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of states within primary market | State	7
Carrying amount	$ 23,289.0	$ 22,153.0
Reclassification from non-accretable difference	$ 97.0	266.5
Sustained period of delinquency for impairment evaluation	90 days
Valuation for impairment of loans with pooled reserves	$ 1.0
Minimum amount to allocate specific valuation allowance	1.0
Interest income on impaired loans still accruing	5.9	5.9	$ 6.1
Restructured loans returned to performing status	4.4
Minimum corporation reserves for commercial loan	1.0
Reserves in allowance for loan losses	196.0	180.0	$ 175.0	$ 158.0
Pooled reserves for all other classes of loans	4.1	4.0
Purchased credit-impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount	$ 1.5	$ 1.7
Loan portfolio diversification risk | Residential construction loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of portfolio	1.30%	1.20%
Credit concentration risk | Acquired loans receivable | Purchased credit-impaired loans | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of portfolio	0.05%
Credit concentration risk | Acquired loans receivable | Purchased credit-impaired loans | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of portfolio		0.04%
Total commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Threshold period past due for default non-accrual status of trade accounts receivable	90 days
Installment Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Threshold period past due for default non-accrual status of trade accounts receivable	120 days
Residential Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Threshold period past due for default non-accrual status of trade accounts receivable	180 days
Carrying value of OREO through foreclosure	$ 3.3	$ 6.3
Mortgage loans on real estate, foreclosure	9.2	8.9
Total commercial loans and leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount	14,721.0	13,761.0
Specific reserves for commercial troubled debt restructurings | Total commercial loans and leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reserves in allowance for loan losses	0.5	0.5
Pooled reserves for individual commercial loans | Total commercial loans and leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reserves in allowance for loan losses	0.8	0.5
Residential construction loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount	$ 297.3	$ 273.4