Loans - Analysis of Information Pertaining to Impaired Loans, Excluding Purchased Non-Covered and Covered 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
Excluding Purchased Non-Covered and Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	41,484	58,035	45,415
Recorded Investment With No Allowance	5,151		8,935
Recorded Investment With Allowance	28,625	45,577	25,552
Total impaired loans	33,776	45,577	34,487
Related Allowance	4,909	4,324	4,008
Average Recorded Investment	34,124	45,997	41,197
Excluding Purchased Non-Covered and Covered Loans [Member] \| Commercial, Financial & Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	2,378	5,421	3,387
Recorded Investment With No Allowance	5		6
Recorded Investment With Allowance	1,287	3,719	1,956
Total impaired loans	1,292	3,719	1,962
Related Allowance	240	394	395
Average Recorded Investment	1,627	4,169	3,021
Excluding Purchased Non-Covered and Covered Loans [Member] \| Real Estate - Construction & Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	7,397	10,636	8,325
Recorded Investment With No Allowance	274		448
Recorded Investment With Allowance	3,801	6,033	4,005
Total impaired loans	4,075	6,033	4,453
Related Allowance	667	736	771
Average Recorded Investment	4,264	5,950	5,368
Excluding Purchased Non-Covered and Covered Loans [Member] \| Commercial And Farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	16,980	19,983	17,514
Recorded Investment With No Allowance	3,280		4,967
Recorded Investment With Allowance	11,922	17,282	9,651
Total impaired loans	15,202	17,282	14,618
Related Allowance	2,127	1,972	1,859
Average Recorded Investment	14,909	16,380	15,972
Excluding Purchased Non-Covered and Covered Loans [Member] \| Real Estate - Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	14,181	21,307	15,571
Recorded Investment With No Allowance	1,592		3,514
Recorded Investment With Allowance	11,166	17,996	9,407
Total impaired loans	12,758	17,996	12,921
Related Allowance	1,869	1,211	974
Average Recorded Investment	12,833	18,983	16,317
Excluding Purchased Non-Covered and Covered Loans [Member] \| Consumer Installment Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	548	688	618
Recorded Investment With Allowance	449	547	533
Total impaired loans	449	547	533
Related Allowance	6	11	9
Average Recorded Investment	$ 491	$ 515	$ 519

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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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+ 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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