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Loans and Allowance for Credit Losses - Outstanding Balances of Loan (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | $ 5,407,376 | $ 4,879,347 | $ 4,683,750
Commercial, financial, agricultural and other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 1,163,383 | 1,139,547 |
Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 248,868 | 219,621 |
Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 1,426,370 | 1,229,192 |
Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 2,019,096 | 1,742,210 |
Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 549,659 | 548,777 |
Originated Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 5,007,361 | 4,778,471 |
Originated Loans [Member] | Commercial, financial, agricultural and other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 1,122,741 | 1,131,148 |
Originated Loans [Member] | Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 242,905 | 217,840 |
Originated Loans [Member] | Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 1,206,119 | 1,165,851 |
Originated Loans [Member] | Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 1,892,185 | 1,717,043 |
Originated Loans [Member] | Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 543,411 | 546,589 |
Acquired Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 400,015 | 100,876 |
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 40,642 | 8,399 |
Acquired Loans [Member] | Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 5,963 | 1,781 |
Acquired Loans [Member] | Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 220,251 | 63,341 |
Acquired Loans [Member] | Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 126,911 | 25,167 |
Acquired Loans [Member] | Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | $ 6,248 | $ 2,188 |

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- Definition Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524 + Details
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