Loans and Allowance for Loan Losses (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Summary of loans outstanding by category
Total loans	$ 3,065,439	$ 3,013,498	$ 2,861,844
Percent	100.00%	100.00%	100.00%
Loans held for sale (included above)	16,612	12,126	11,258
Commercial and Industrial [Member]
Summary of loans outstanding by category
Total loans	515,456	547,942	551,293
Percent	16.82%	18.19%	19.26%
Oil & gas production & equipment [Member]
Summary of loans outstanding by category
Total loans	125,228	115,786	113,868
Percent	4.08%	3.84%	3.98%
Agriculture [Member]
Summary of loans outstanding by category
Total loans	77,882	86,297	74,221
Percent	2.54%	2.86%	2.59%
Construction [Member]
Summary of loans outstanding by category
Total loans	197,168	207,953	236,660
Percent	6.43%	6.90%	8.27%
Farmland [Member]
Summary of loans outstanding by category
Total loans	111,472	103,923	86,285
Percent	3.64%	3.45%	3.02%
One to four family residences [Member]
Summary of loans outstanding by category
Total loans	674,577	655,134	618,428
Percent	22.01%	21.74%	21.61%
Multifamily residential properties [Member]
Summary of loans outstanding by category
Total loans	46,866	37,734	34,040
Percent	1.53%	1.25%	1.19%
Taxable [Member]
Summary of loans outstanding by category
Total loans	6,520	6,939	7,281
Percent	0.21%	0.23%	0.25%
Tax-exempt [Member]
Summary of loans outstanding by category
Total loans	13,853	17,070	11,920
Percent	0.45%	0.57%	0.42%
Commercial [Member]
Summary of loans outstanding by category
Total loans	1,036,322	960,074	846,684
Percent	33.81%	31.86%	29.59%
Consumer [Member]
Summary of loans outstanding by category
Total loans	239,156	252,331	255,975
Percent	7.80%	8.37%	8.94%
Other (not classified above) [Member]
Summary of loans outstanding by category
Total loans	$ 20,939	$ 22,315	$ 25,189
Percent	0.68%	0.74%	0.88%