Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Summary of Impaired Loans, Including Restructured Loans) (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Receivables [Abstract]
Impaired loans with an allowance for loan loss required	$ 52,485	$ 60,219	$ 83,349
Impaired loans with no allowance for loan loss required	78,338	67,050	49,173
Total impaired loans	130,823	127,269	132,522
Allowance for loan losses related to impaired loans	14,934	11,437	14,365
TDRs	$ 66,308	$ 66,102	$ 66,219

Amount of financing receivable modified as troubled debt restructuring.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

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-Name Accounting Standards Codification
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-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=118939409&loc=d3e18845-111554

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Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

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-Publisher FASB
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-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
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Amount of allowance for credit losses related to recorded investment.

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Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

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Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

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