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Loans and Allowance for Loan Losses - Internal Loan Grading by Class of Loans (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | $ 5,007,518 | $ 4,721,995
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 639,618 | 641,618
Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,129,730 | 1,126,591
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 329,683 | 330,334
Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 794,355 | 785,655
Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,304,414 | 1,284,405
Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 301,127 | 288,920
Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 137,351 | 144,445
Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 371,240 | 120,027
Grade 1 [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 3,982,551 | 3,851,837
Grade 1 [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 492,984 | 520,641
Grade 1 [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 929,230 | 931,295
Grade 1 [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 282,347 | 289,200
Grade 1 [Member] | Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 622,575 | 621,401
Grade 1 [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,029,512 | 1,018,172
Grade 1 [Member] | Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 278,585 | 268,826
Grade 1 [Member] | Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 131,903 | 136,617
Grade 1 [Member] | Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 215,415 | 65,685
Grade 2 [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 891,154 | 758,681
Grade 2 [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 126,220 | 105,696
Grade 2 [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 181,759 | 178,282
Grade 2 [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 37,534 | 33,033
Grade 2 [Member] | Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 154,192 | 149,201
Grade 2 [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 249,253 | 234,884
Grade 2 [Member] | Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 20,078 | 17,499
Grade 2 [Member] | Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 5,054 | 5,668
Grade 2 [Member] | Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 117,064 | 34,418
Grade 3 [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 100,001 | 88,588
Grade 3 [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 18,995 | 13,852
Grade 3 [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 9,701 | 14,290
Grade 3 [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 7,350 | 6,352
Grade 3 [Member] | Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 10,681 | 9,418
Grade 3 [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 18,716 | 24,322
Grade 3 [Member] | Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,675 | 2,038
Grade 3 [Member] | Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 336 | 1,203
Grade 3 [Member] | Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 32,547 | 17,113
Grade 4 [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 33,812 | 22,859
Grade 4 [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,419 | 1,429
Grade 4 [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 9,040 | 2,724
Grade 4 [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 2,452 | 1,749
Grade 4 [Member] | Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 6,907 | 5,635
Grade 4 [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 6,933 | 6,997
Grade 4 [Member] | Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 789 | 557
Grade 4 [Member] | Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 58 | 957
Grade 4 [Member] | Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | $ 6,214 | 2,811
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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