Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	$ 4,223,078	$ 4,157,470
Commercial real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	1,447,145	1,497,272
Land acquisition & development construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	210,761	220,196
Residential construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	60,883	49,274
Commercial construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	65,567	65,059
Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	337,211	334,529
Residential real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	804,200	708,339
Agricultural real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	176,799	177,244
Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	2,765,355	2,717,384
Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	457,151	438,245
Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	136,459	137,743
Credit card consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	59,334	60,806
Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	652,944	636,794
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	680,751	688,753
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	121,530	113,627
Other Assets Especially Mentioned
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	192,390	209,932
Other Assets Especially Mentioned | Commercial real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	102,975	101,936
Other Assets Especially Mentioned | Land acquisition & development construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	25,726	28,137
Other Assets Especially Mentioned | Residential construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	2,363	2,531
Other Assets Especially Mentioned | Commercial construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	0	3,000
Other Assets Especially Mentioned | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	28,089	33,668
Other Assets Especially Mentioned | Residential real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	6,834	9,542
Other Assets Especially Mentioned | Agricultural real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	12,397	18,490
Other Assets Especially Mentioned | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	150,295	163,636
Other Assets Especially Mentioned | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	757	793
Other Assets Especially Mentioned | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	586	684
Other Assets Especially Mentioned | Credit card consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	0	0
Other Assets Especially Mentioned | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	1,343	1,477
Other Assets Especially Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	37,458	42,223
Other Assets Especially Mentioned | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	3,294	2,596
Substandard
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	161,786	215,188
Substandard | Commercial real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	101,713	135,282
Substandard | Land acquisition & development construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	16,687	25,884
Substandard | Residential construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	1,530	2,427
Substandard | Commercial construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	485	795
Substandard | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	18,702	29,106
Substandard | Residential real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	7,439	11,680
Substandard | Agricultural real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	11,251	6,737
Substandard | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	139,105	182,805
Substandard | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	1,517	1,764
Substandard | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	782	1,395
Substandard | Credit card consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	450	415
Substandard | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	2,749	3,574
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	18,437	27,184
Substandard | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	1,495	1,625
Doubtful
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	52,266	42,460
Doubtful | Commercial real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	18,377	15,173
Doubtful | Land acquisition & development construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	6,733	4,739
Doubtful | Residential construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	279	1,143
Doubtful | Commercial construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	5,707	7,383
Doubtful | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	12,719	13,265
Doubtful | Residential real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	5,867	4,511
Doubtful | Agricultural real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	2,558	3,228
Doubtful | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	39,521	36,177
Doubtful | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	124	114
Doubtful | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	386	628
Doubtful | Credit card consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	2,291	2,085
Doubtful | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	2,801	2,827
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	9,922	3,428
Doubtful | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	22	28
Total Criticized Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	406,442	467,580
Total Criticized Loans | Commercial real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	223,065	252,391
Total Criticized Loans | Land acquisition & development construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	49,146	58,760
Total Criticized Loans | Residential construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	4,172	6,101
Total Criticized Loans | Commercial construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	6,192	11,178
Total Criticized Loans | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	59,510	76,039
Total Criticized Loans | Residential real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	20,140	25,733
Total Criticized Loans | Agricultural real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	26,206	28,455
Total Criticized Loans | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	328,921	382,618
Total Criticized Loans | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	2,398	2,671
Total Criticized Loans | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	1,754	2,707
Total Criticized Loans | Credit card consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	2,741	2,500
Total Criticized Loans | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	6,893	7,878
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	65,817	72,835
Total Criticized Loans | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	$ 4,811	$ 4,249