Loans and Allowance for Loan Losses - Summary of Impaired Financial Receivables (Analysis of Covered Impaired Loans) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	$ 2,835,399	$ 2,457,444	$ 1,958,347
Commercial, Financial & Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	319,654	244,373	174,217
Real Estate - Construction & Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	161,507	146,371	114,199
Commercial And Farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	907,524	808,323	732,322
Real Estate - Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	456,106	366,882	346,480
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	65,555	106,690	507,712
Recorded Investment With No Allowance	58,031	91,395
Recorded Investment With Allowance	0	0
Total Recorded Investment	58,031	91,395
Related Allowance	0	0
Average Recorded Investment	73,757	110,830
Covered Loans [Member] \| Commercial, Financial & Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	10,845	9,680
Recorded Investment With No Allowance	8,582	7,270
Recorded Investment With Allowance	0	0
Total Recorded Investment	8,582	7,270
Related Allowance	0	0
Average Recorded Investment	9,777	8,696
Covered Loans [Member] \| Real Estate - Construction & Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	11,621	20,915
Recorded Investment With No Allowance	10,638	18,037
Recorded Investment With Allowance	0	0
Total Recorded Investment	10,638	18,037
Related Allowance	0	0
Average Recorded Investment	14,132	21,794
Covered Loans [Member] \| Commercial And Farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	23,349	46,612
Recorded Investment With No Allowance	20,663	40,749
Recorded Investment With Allowance	0	0
Total Recorded Investment	20,663	40,749
Related Allowance	0	0
Average Recorded Investment	28,594	51,584
Covered Loans [Member] \| Real Estate - Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	19,629	29,089
Recorded Investment With No Allowance	18,054	24,998
Recorded Investment With Allowance	0	0
Total Recorded Investment	18,054	24,998
Related Allowance	0	0
Average Recorded Investment	21,091	28,452
Covered Loans [Member] \| Consumer Installment Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	111	394
Recorded Investment With No Allowance	94	341
Recorded Investment With Allowance	0	0
Total Recorded Investment	94	341
Related Allowance	0	0
Average Recorded Investment	$ 163	$ 304

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Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5258-111524

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Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 14A

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953550-111524

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+ References

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+ References

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+ References

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