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Loans and Allowance for Loan Losses - Internal Loan Grading by Class of Loans (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | $ 4,721,995 | $ 4,400,232
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 641,618 | 571,631
Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,126,591 | 1,123,396
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 330,334 | 325,413
Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 785,655 | 748,229
Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,284,405 | 1,051,185
Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 288,920 | 283,877
Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 144,445 | 141,482
Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 120,027 | 155,019
Grade 1 [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 3,851,837 | 3,642,438
Grade 1 [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 520,641 | 464,504
Grade 1 [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 931,295 | 933,743
Grade 1 [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 289,200 | 284,893
Grade 1 [Member] | Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 621,401 | 614,338
Grade 1 [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,018,172 | 856,318
Grade 1 [Member] | Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 268,826 | 263,442
Grade 1 [Member] | Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 136,617 | 132,254
Grade 1 [Member] | Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 65,685 | 92,946
Grade 2 [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 758,681 | 647,509
Grade 2 [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 105,696 | 89,978
Grade 2 [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 178,282 | 169,561
Grade 2 [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 33,033 | 32,889
Grade 2 [Member] | Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 149,201 | 119,018
Grade 2 [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 234,884 | 170,865
Grade 2 [Member] | Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 17,499 | 17,154
Grade 2 [Member] | Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 5,668 | 5,376
Grade 2 [Member] | Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 34,418 | 42,668
Grade 3 [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 88,588 | 82,136
Grade 3 [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 13,852 | 16,220
Grade 3 [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 14,290 | 14,404
Grade 3 [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 6,352 | 5,987
Grade 3 [Member] | Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 9,418 | 9,382
Grade 3 [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 24,322 | 19,101
Grade 3 [Member] | Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 2,038 | 2,640
Grade 3 [Member] | Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,203 | 1,514
Grade 3 [Member] | Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 17,113 | 12,888
Grade 4 [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 22,859 | 28,149
Grade 4 [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,429 | 929
Grade 4 [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 2,724 | 5,688
Grade 4 [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,749 | 1,644
Grade 4 [Member] | Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 5,635 | 5,491
Grade 4 [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 6,997 | 4,901
Grade 4 [Member] | Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 557 | 641
Grade 4 [Member] | Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 957 | 2,338
Grade 4 [Member] | Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 2,811 | $ 6,517
Grade 5 [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 30 |
Grade 5 [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | $ 30 |

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