Loans and Allowance for Loan Losses - Additional Information (Detail) - USD ($)	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan placed on nonaccrual when past due, number of days	90 days
Loan participations	$ 952,500,000
Loan participations sold	471,600,000
Loan participations retained	$ 480,900,000
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	$ 27,000,000.0		$ 26,300,000
Financing receivable troubled debt restructurings specific reserves	3,800,000		3,000,000.0
Unfunded commitments on TDRs	5,000,000.0
Increase (decrease) in allowance for loan losses	0	$ 0
Allowance for loan losses charge-offs	$ 0	0
Number of days for a loan to be considered to be in payment default	90 days
Outstanding loans including principal, interest, fees and penalties	$ 243,700,000		246,900,000
Accretion of income	2,000,000.0	$ 4,500,000
Financing receivable allowance for loan losses related to purchased loans	313,000		478,000
Maximum [Member]
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 [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Percentage of risk-based capital	194.00%
Regulatory guideline limit	300.00%
Financing receivable allowance for loan losses related to purchased loans	$ 153,000		$ 319,000
Credit Concentration Risk [Member] \| Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Concentration Risk Percentage	10.00%
Credit Concentration Risk [Member] \| Non Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Concentration Risk Percentage	10.00%

Certain loans acquired in transfer not accounted for as debt securities outstanding balance1.

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- Definition

Commercial real estate loans as a percentage of risk-based capital.

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The regulatory guideline limit for commercial real estate loans as a percentage of risk-based capital.

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- Definition

Maximum number of days past due before loan is placed on nonaccrual status.

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Amount of troubled debt restructured loans that are on non accrual status.

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Amount of allowance for loan losses associated with troubled debt restructured loans.

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Maximum number of days past due before loan is charged off.

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Minimum number of days past due that loan is charged off.

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Amount of loan participations with other financial institutions.

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Amount of loan participations retained.

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Amount of loan participations sold to other financial institutions.

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Number of days contractually past due under the modified terms that a troubled debt restructured loan is considered to be in payment default.

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Number of days delinquent without regard to collateral position for small commercial loans.

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Amount of time a borrower of a troubled debt restructuring loan must perform in accordance with the restructured terms before returning to accrual status.

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Current period recognition due to accretion of the excess of a loan's cash flows expected to be collected over the investor's initial investment in the loan for loans accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

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Reserve for loans impairments recorded after acquisition as a result of current information and events which result in it being probable that estimated cash flows will be less than previously expected or because actual cash flows are less than cash flows previously expected. This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that was acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The increase (decrease) in the allowance for credit losses related to financing receivables.

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Amount of direct write-downs of financing receivables charged against the allowance.

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Reflects the amount of the write-down related to a troubled debt restructuring.

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Reflects the unfunded portion of contractual agreements to lend funds to borrowers for loans whose terms have been modified by troubled debt restructuring.

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