Allowance for Loan and Lease Losses and Recorded Investment in Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Allowance for loan and lease losses
Balance at beginning of period	$ 52,553	$ 40,279	$ 42,156	$ 47,931
Charge-offs	(4,756)	(3,444)	(12,022)	(20,259)
Recoveries	138	282	501	945
Provision charged to operating expense	9,500	6,000	26,800	14,500
Balance at end of period	57,435	43,117	57,435	43,117
Period-end amount allocated to:
Loans individually evaluated for impairment	33,756	13,641	33,756	13,641	7,315
Other loans not individually evaluated	23,679	29,476	23,679	29,476	34,841
Ending balance	57,435	43,117	57,435	43,117	42,156
Loans individually evaluated for impairment	144,872	110,443	144,872	110,443	96,423
Other loans not individually evaluated	1,039,270	1,197,143	1,039,270	1,197,143	1,180,667
Ending balance	1,184,142	1,307,586	1,184,142	1,307,586	1,277,090
Commercial, financial and agricultural
Allowance for loan and lease losses
Balance at beginning of period	4,591	4,569	5,151	5,429
Charge-offs	(305)	(35)	(1,354)	(2,271)
Recoveries	57	158	108	229
Provision charged to operating expense	1,117	591	1,555	1,896
Balance at end of period	5,460	5,283	5,460	5,283
Period-end amount allocated to:
Loans individually evaluated for impairment	1,625	208	1,625	208	206
Other loans not individually evaluated	3,835	5,075	3,835	5,075	4,945
Ending balance	5,460	5,283	5,460	5,283	5,151
Loans individually evaluated for impairment	6,237	1,396	6,237	1,396	1,393
Other loans not individually evaluated	270,542	290,985	270,542	290,985	290,481
Ending balance	276,779	292,381	276,779	292,381	291,874
Commercial real estate
Allowance for loan and lease losses
Balance at beginning of period	37,960	26,298	23,280	31,431
Charge-offs	(3,857)	(3,033)	(7,227)	(15,930)
Recoveries	19	48	49	520
Provision charged to operating expense	5,769	4,623	23,789	11,915
Balance at end of period	39,891	27,936	39,891	27,936
Period-end amount allocated to:
Loans individually evaluated for impairment	29,008	11,273	29,008	11,273	4,779
Other loans not individually evaluated	10,883	16,663	10,883	16,663	18,501
Ending balance	39,891	27,936	39,891	27,936	23,280
Loans individually evaluated for impairment	124,017	93,003	124,017	93,003	79,557
Other loans not individually evaluated	495,400	602,797	495,400	602,797	595,992
Ending balance	619,417	695,800	619,417	695,800	675,549
Residential real estate
Allowance for loan and lease losses
Balance at beginning of period	8,168	6,430	7,713	6,669
Charge-offs	(483)	(328)	(3,195)	(1,902)
Recoveries	29	33	254	79
Provision charged to operating expense	924	991	3,866	2,280
Balance at end of period	8,638	7,126	8,638	7,126
Period-end amount allocated to:
Loans individually evaluated for impairment	3,063	1,835	3,063	1,835	2,270
Other loans not individually evaluated	5,575	5,291	5,575	5,291	5,444
Ending balance	8,638	7,126	8,638	7,126	7,714
Loans individually evaluated for impairment	14,499	15,658	14,499	15,658	15,354
Other loans not individually evaluated	227,713	250,147	227,713	250,147	243,335
Ending balance	242,212	265,805	242,212	265,805	258,689
Consumer Portfolio Segment
Allowance for loan and lease losses
Balance at beginning of period	420	887	584	890
Charge-offs	(111)	(48)	(246)	(156)
Recoveries	33	43	90	117
Provision charged to operating expense	10	(3)	(76)	28
Balance at end of period	352	879	352	879
Period-end amount allocated to:
Loans individually evaluated for impairment	60	325	60	325	60
Other loans not individually evaluated	292	554	292	554	523
Ending balance	352	879	352	879	583
Loans individually evaluated for impairment	119	386	119	386	119
Other loans not individually evaluated	45,615	53,214	45,615	53,214	50,859
Ending balance	45,734	53,600	45,734	53,600	50,978
Unallocated
Allowance for loan and lease losses
Balance at beginning of period	1,414	2,095	5,428	3,512
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision charged to operating expense	1,680	(202)	(2,334)	(1,619)
Balance at end of period	3,094	1,893	3,094	1,893
Period-end amount allocated to:
Loans individually evaluated for impairment	0	0	0	0	0
Other loans not individually evaluated	3,094	1,893	3,094	1,893	5,428
Ending balance	3,094	1,893	3,094	1,893	5,428
Loans individually evaluated for impairment	0	0	0	0	0
Other loans not individually evaluated	0	0	0	0	0
Ending balance	$ 0	$ 0	$ 0	$ 0	$ 0