Loans - Summary of Impaired Financial Receivables (Analysis of Covered Impaired Loans) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	$ 2,888,257	$ 2,505,345	$ 2,835,399
Commercial, Financial & Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	334,917	270,571	319,654
Real Estate - Construction & Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	178,568	149,543	161,507
Commercial And Farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	947,274	836,230	907,524
Real Estate - Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	496,043	393,001	456,106
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	97,212	164,467	95,967
Recorded Investment With No Allowance	59,080	82,556	58,031
Recorded Investment With Allowance	0	0	0
Total Recorded Investment	59,080	82,556	58,031
Related Allowance	0	0	0
Average Recorded Investment	58,556	86,976	73,757
Covered Loans [Member] \| Commercial, Financial & Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	13,512	16,020	14,385
Recorded Investment With No Allowance	8,407	10,039	8,582
Recorded Investment With Allowance	0	0	0
Total Recorded Investment	8,407	10,039	8,582
Related Allowance	0	0	0
Average Recorded Investment	8,495	8,655	9,777
Covered Loans [Member] \| Real Estate - Construction & Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	24,503	50,876	27,289
Recorded Investment With No Allowance	9,080	18,034	10,638
Recorded Investment With Allowance	0	0	0
Total Recorded Investment	9,080	18,034	10,638
Related Allowance	0	0	0
Average Recorded Investment	9,859	18,036	14,132
Covered Loans [Member] \| Commercial And Farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	35,493	66,557	31,309
Recorded Investment With No Allowance	23,462	31,746	20,663
Recorded Investment With Allowance	0	0	0
Total Recorded Investment	23,462	31,746	20,663
Related Allowance	0	0	0
Average Recorded Investment	22,062	36,247	28,594
Covered Loans [Member] \| Real Estate - Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	23,585	30,824	22,860
Recorded Investment With No Allowance	18,042	22,604	18,054
Recorded Investment With Allowance	0	0	0
Total Recorded Investment	18,042	22,604	18,054
Related Allowance	0	0	0
Average Recorded Investment	18,048	23,801	21,091
Covered Loans [Member] \| Consumer Installment Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	119	190	124
Recorded Investment With No Allowance	89	133	94
Recorded Investment With Allowance	0	0	0
Total Recorded Investment	89	133	94
Related Allowance	0	0	0
Average Recorded Investment	$ 92	$ 237	$ 163

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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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