Loans and Allowance for Credit Losses - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2020 USD ($) portfolio	Mar. 31, 2020 numberOfLoanPortfolios	Mar. 31, 2020	Mar. 31, 2020 segment	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
Number of loan portfolios | portfolio	4
Number of loan segments		7		7
Accrued interest receivable, loans					$ 54,000,000.0
Loan placed on nonaccrual when past due, number of days	90 days
Loan participations					849,400,000
Loan participations sold					401,600,000
Loan participations retained					447,800,000
Troubled debt restructuring 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
Nonaccrual period for loans	90 days
Financing receivable TDR's included with non-accrual loans					11,800,000	$ 13,800,000
Financing receivable troubled debt restructurings specific reserves					1,400,000	900,000
Unfunded commitments on TDRs					$ 900,000	$ 2,300,000
Number of days for a loan to be considered to be in payment default	90 days
Maximum
Financing Receivable, Allowance for Credit Losses [Line Items]
Value of small commercial loans on nonaccrual status or 90 days or more delinquent	$ 250,000
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Percentage of risk-based capital			206.00%
Regulatory guideline limit			300.00%