Loans - Analysis of Information Pertaining to Purchased Non-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
Purchased Non-Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	29,616	35,136	33,989
Recorded Investment With No Allowance	18,834	20,509	19,461
Recorded Investment With Allowance	0	0	0
Total Recorded Investment	18,834	20,509	19,461
Related Allowance	0	0	0
Average Recorded Investment	19,148	16,553	16,444
Purchased Non-Covered Loans [Member] \| Commercial, Financial & Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	1,331	233	1,366
Recorded Investment With No Allowance	198	117	175
Recorded Investment With Allowance	0	0	0
Total Recorded Investment	198	117	175
Related Allowance	0	0	0
Average Recorded Investment	187	64	165
Purchased Non-Covered Loans [Member] \| Real Estate - Construction & Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	2,153	6,173	5,161
Recorded Investment With No Allowance	1,113	3,574	1,436
Recorded Investment With Allowance	0	0	0
Total Recorded Investment	1,113	3,574	1,436
Related Allowance	0	0	0
Average Recorded Investment	1,275	3,631	1,643
Purchased Non-Covered Loans [Member] \| Commercial And Farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	13,911	12,966	15,007
Recorded Investment With No Allowance	9,816	7,790	10,588
Recorded Investment With Allowance	0	0	0
Total Recorded Investment	9,816	7,790	10,588
Related Allowance	0	0	0
Average Recorded Investment	10,202	5,336	7,484
Purchased Non-Covered Loans [Member] \| Real Estate - Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	12,183	15,524	12,283
Recorded Investment With No Allowance	7,679	8,987	7,191
Recorded Investment With Allowance	0	0	0
Total Recorded Investment	7,679	8,987	7,191
Related Allowance	0	0	0
Average Recorded Investment	7,435	7,483	7,084
Purchased Non-Covered Loans [Member] \| Consumer Installment Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	38	240	172
Recorded Investment With No Allowance	28	41	71
Recorded Investment With Allowance	0	0	0
Total Recorded Investment	28	41	71
Related Allowance	0	0	0
Average Recorded Investment	$ 50	$ 39	$ 68

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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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- Definition

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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- Definition

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+ References

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- Definition

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+ References

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- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

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- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

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