Loans Receivable and Allowance for Credit Losses on Loans (Narrative) (Detail)			3 Months Ended		12 Months Ended
	Apr. 28, 2020 USD ($) counterparty	Jan. 01, 2020 USD ($)	Mar. 31, 2020 USD ($) SecurityLoan troubled_debt_restructuring borrower	Mar. 31, 2019 USD ($) SecurityLoan	Dec. 31, 2019 USD ($) SecurityLoan borrower troubled_debt_restructuring counterparty	Dec. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less allowance for credit losses		$ 55,525,000	$ 75,143,000	$ 55,353,000	$ 55,525,000	$ 55,562,000
Retained earnings			686,397,000		695,273,000
Principal amount of nonaccrual loans			35,300,000		40,200,000
Loans less than 90 days past due			$ 16,000,000.0		$ 13,100,000
Number of loans 90 days past due and still accruing | SecurityLoan			0		0
Impaired loan defined floor limit (greater than)			$ 1,000,000.0
Impaired loans number | SecurityLoan			158		158
Impaired loans			$ 65,732,000		$ 70,641,000
Number of troubled debt restructurings | troubled_debt_restructuring			129		147
Number of borrowers | borrower			125		128,000,000
Loans and leases receivable, impaired, nonperforming, accrual of interest			$ 39,100,000		$ 42,700,000
Troubled debt restructuring loans					$ 48,300,000
Number of troubled debt restructurings with restructured terms | counterparty					133,000,000
Collateral dependent impaired loans			15,900,000	20,400,000
Provision for credit losses			14,717,000	$ 200,000
Charge off, impaired loan			$ 2,700,000
Weighted average modified interest rate			6.99%
Weighted average prior modification rate			7.19%
Number of payment defaults for loans modified as TDRs | SecurityLoan			0	3
Purchased credit-impaired loans					$ (746,000)
Allowances for loan losses			$ 5,699,000		5,067,000
Impaired financing receivable with no related allowance			27,600,000		16,000,000.0
Average balance of impaired loans			76,972,000		$ 74,137,000
Commercial Loan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent impaired loans			12,800,000
Residential Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent impaired loans			3,000,000.0
Consumer Loan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent impaired loans			174,000
Subsequent Event
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of borrowers | counterparty	638
Number of PPP loans | counterparty	820
Principal loan balances deferred	$ 889,000,000.0
Paycheck protection program	$ 377,500,000
Team Capital Bank
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance of PCI loans			737,000
Impaired Loans Troubled Debt Restructurings
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Allowances for loan losses			448,000
Accounting Standards Update 2016-13
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less allowance for credit losses		7,900,000
Retained earnings		8,300,000
Provision for credit losses		1,000,000.0	$ 17,000
Accounting Standards Update 2016-13, Effect On Loans Receivable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Retained earnings		$ 5,900,000