Allowance for Loan Losses - Investment in Loans, Net of Unearned Income on Impairment Methodology (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment in loans, net of unearned income on impairment methodology
Individually evaluated for impairment	$ 25,206	$ 30,307
Collectively evaluated for impairment	7,458,554	7,362,755
Loans, net of unearned income	7,698,070	7,620,322
Receivables Acquired with Deteriorated Credit Quality
Investment in loans, net of unearned income on impairment methodology
Purchased with deteriorated credit quality	214,310	227,260
Commercial, financial, agricultural
Investment in loans, net of unearned income on impairment methodology
Individually evaluated for impairment	2,848	3,064
Collectively evaluated for impairment	1,031,037	1,021,014
Loans, net of unearned income	1,046,818	1,039,393
Commercial, financial, agricultural \| Receivables Acquired with Deteriorated Credit Quality
Investment in loans, net of unearned income on impairment methodology
Purchased with deteriorated credit quality	12,933	15,315
Real estate – construction
Investment in loans, net of unearned income on impairment methodology
Individually evaluated for impairment	399	1,777
Collectively evaluated for impairment	657,092	631,612
Loans, net of unearned income	657,491	633,389
Real estate – construction \| Receivables Acquired with Deteriorated Credit Quality
Investment in loans, net of unearned income on impairment methodology
Purchased with deteriorated credit quality	0	0
Real estate – 1-4 family mortgage
Investment in loans, net of unearned income on impairment methodology
Individually evaluated for impairment	13,737	14,482
Collectively evaluated for impairment	2,294,168	2,275,270
Loans, net of unearned income	2,358,101	2,343,721
Real estate – 1-4 family mortgage \| Receivables Acquired with Deteriorated Credit Quality
Investment in loans, net of unearned income on impairment methodology
Purchased with deteriorated credit quality	50,196	53,969
Real estate – commercial mortgage
Investment in loans, net of unearned income on impairment methodology
Individually evaluated for impairment	7,802	10,545
Collectively evaluated for impairment	3,306,604	3,260,648
Loans, net of unearned income	3,463,953	3,427,530
Real estate – commercial mortgage \| Receivables Acquired with Deteriorated Credit Quality
Investment in loans, net of unearned income on impairment methodology
Purchased with deteriorated credit quality	149,547	156,337
Installment loans to individuals
Investment in loans, net of unearned income on impairment methodology
Individually evaluated for impairment	420	439
Collectively evaluated for impairment	169,653	174,211
Loans, net of unearned income	171,707	176,289
Installment loans to individuals \| Receivables Acquired with Deteriorated Credit Quality
Investment in loans, net of unearned income on impairment methodology
Purchased with deteriorated credit quality	$ 1,634	$ 1,639

Investment in loans, net of unearned income on impairment methodology.

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- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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The balance of financing receivables that were individually evaluated for impairment.

+ References

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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

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- Definition

Net amount of the investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, credit card receivables, notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Excludes trade accounts receivable with contractual maturity of one year or less and arose from the sale of goods or services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.3)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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