Fair Value Measurements - Additional Information about Assets Measured at Fair Value on Non-Recurring Basis (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans	$ 45.0
Impaired loans	13.2	$ 5.0
Other real estate owned	3.0	4.0
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0.0	0.0
Level 1 | Fair value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans	0.0
Impaired loans		0.0
Other real estate owned	0.0	0.0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0.0	0.0
Level 2 | Fair value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans	0.0
Impaired loans		0.0
Other real estate owned	0.0	0.0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	39.0	48.0
Level 3 | Fair value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans	45.0
Impaired loans		5.0
Other real estate owned	3.0	4.0
SBA-Guaranteed Loan Servicing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	3.0	3.0	$ 4.0	$ 5.0
SBA-Guaranteed Loan Servicing | Fair value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	3.0	3.0
SBA-Guaranteed Loan Servicing | Level 1 | Fair value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0.0	0.0
SBA-Guaranteed Loan Servicing | Level 2 | Fair value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0.0	0.0
SBA-Guaranteed Loan Servicing | Level 3 | Fair value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	3.0	3.0
Mortgage Servicing Rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	35.6	45.2	$ 41.1	$ 32.4
Mortgage Servicing Rights | Fair value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	36.0	30.0
Mortgage Servicing Rights | Level 1 | Fair value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0.0	0.0
Mortgage Servicing Rights | Level 2 | Fair value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0.0	0.0
Mortgage Servicing Rights | Level 3 | Fair value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	$ 36.0	$ 30.0