Loans Receivable and Allowance for Credit Losses (Narrative) (Detail)	3 Months Ended		6 Months Ended		9 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($) SecurityLoan counterparty	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) SecurityLoan troubled_debt_restructuring counterparty borrower	Jun. 30, 2019 USD ($) SecurityLoan	Sep. 30, 2019 borrower troubled_debt_restructuring	Dec. 31, 2019 USD ($) SecurityLoan	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less allowance for credit losses	$ 86,259,000	$ 62,810,000	$ 86,259,000	$ 62,810,000		$ 55,525,000	$ 75,143,000	$ 55,525,000	$ 55,353,000	$ 55,562,000
Retained earnings	685,509,000		685,509,000			695,273,000
Principal amount of nonaccrual loans	35,500,000		35,500,000			40,200,000
Loans less than 90 days past due	$ 14,800,000		$ 14,800,000			$ 13,100,000
Number of loans 90 days past due and still accruing | SecurityLoan	0		0			0
Impaired loan defined floor limit (greater than)	$ 1,000,000.0		$ 1,000,000.0
Impaired loans number | SecurityLoan			148			158
Impaired loans	63,054,000		$ 63,054,000			$ 70,641,000
Number of troubled debt restructurings | troubled_debt_restructuring			125		147
Number of borrowers | borrower			122		128
Loans and leases receivable, impaired, nonperforming, accrual of interest	37,900,000		$ 37,900,000			42,700,000
Collateral-dependent impaired loans	17,000,000.0		17,000,000.0			20,400,000
Provision for credit losses	10,900,000	9,500,000	25,617,000	9,700,000
Charge off, impaired loan	$ 447,000	$ 2,000,000.0	$ 3,200,000	$ 2,000,000.0
Weighted average modified interest rate	5.33%		6.03%
Weighted average prior modification rate	5.29%		6.07%
Number of payment defaults for loans modified as TDRs | SecurityLoan			0	2
Purchased credit-impaired loans						746,000
Allowances for loan losses	$ 3,625,000		$ 3,625,000			5,067,000
Impaired financing receivable with no related allowance	$ 31,000,000.0		31,000,000.0			16,000,000.0
Average balance of impaired loans			$ 66,637,000			$ 74,137,000
Number of PPP loans | counterparty	1,025		1,025
Paycheck protection program	$ 400,300,000		$ 400,300,000
Consumer Loan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral-dependent impaired loans	9,000		9,000
Commercial Loan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral-dependent impaired loans	15,400,000		15,400,000
Residential Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral-dependent impaired loans	1,500,000		1,500,000
Impaired Loans Troubled Debt Restructurings
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans number | SecurityLoan						133
Loans and leases receivable, impaired, nonperforming, accrual of interest						$ 48,300,000
Allowances for loan losses	$ 9,000		$ 442,000
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
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