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Loans and Allowance for Loan Losses - Internal Loan Grading by Class of Loans (Detail) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | $ 4,984,484 | $ 4,721,995
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 672,584 | 641,618
Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,113,019 | 1,126,591
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 331,050 | 330,334
Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 778,280 | 785,655
Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,256,712 | 1,284,405
Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 284,760 | 288,920
Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 138,677 | 144,445
Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 409,402 | 120,027
Grade 1 [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 3,959,437 | 3,851,837
Grade 1 [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 530,979 | 520,641
Grade 1 [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 912,482 | 931,295
Grade 1 [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 287,363 | 289,200
Grade 1 [Member] | Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 611,885 | 621,401
Grade 1 [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 982,927 | 1,018,172
Grade 1 [Member] | Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 263,827 | 268,826
Grade 1 [Member] | Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 132,305 | 136,617
Grade 1 [Member] | Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 237,669 | 65,685
Grade 2 [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 882,039 | 758,681
Grade 2 [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 121,730 | 105,696
Grade 2 [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 182,611 | 178,282
Grade 2 [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 34,047 | 33,033
Grade 2 [Member] | Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 151,133 | 149,201
Grade 2 [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 244,432 | 234,884
Grade 2 [Member] | Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 18,099 | 17,499
Grade 2 [Member] | Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 5,318 | 5,668
Grade 2 [Member] | Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 124,669 | 34,418
Grade 3 [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 109,938 | 88,588
Grade 3 [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 18,485 | 13,852
Grade 3 [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 8,645 | 14,290
Grade 3 [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 7,562 | 6,352
Grade 3 [Member] | Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 9,587 | 9,418
Grade 3 [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 22,054 | 24,322
Grade 3 [Member] | Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,939 | 2,038
Grade 3 [Member] | Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 873 | 1,203
Grade 3 [Member] | Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 40,793 | 17,113
Grade 4 [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 33,041 | 22,859
Grade 4 [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 1,390 | 1,429
Grade 4 [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 9,281 | 2,724
Grade 4 [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 2,078 | 1,749
Grade 4 [Member] | Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 5,675 | 5,635
Grade 4 [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 7,270 | 6,997
Grade 4 [Member] | Consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 895 | 557
Grade 4 [Member] | Other Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 181 | 957
Grade 4 [Member] | Acquired Loans [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 6,271 | 2,811
Grade 5 [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | 29 | 30
Grade 5 [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Total loans | $ 29 | $ 30

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