Schedule of Allowance for Loan Losses by Portfolio Segment (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 112,581	$ 120,480	$ 103,030
Provision charged to operating expense	40,750	58,151	66,900
Less loans charged-off	(60,936)	(71,281)	(52,339)
Add back recoveries of loans previously charged-off	8,116	5,231	2,889
Ending balance	100,511	112,581	120,480
Allowance for loan losses:
Loans individually evaluated for impairment	10,297	32,838	42,226
Loans collectively evaluated for impairment	90,214	79,743	78,254
Ending balance	100,511	112,581	120,480
Total loans:
Individually evaluated for impairment	136,185	230,596	199,302
Individually evaluated for impairment	4,087,727	3,955,953	4,168,607
Loans held for investment	4,223,912	4,186,549	4,367,909
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	87,396	84,181	76,357
Provision charged to operating expense	28,651	46,844	42,163
Less loans charged-off	(43,506)	(45,764)	(34,718)
Add back recoveries of loans previously charged-off	3,241	2,135	379
Ending balance	75,782	87,396	84,181
Allowance for loan losses:
Loans individually evaluated for impairment	8,350	28,023	27,081
Loans collectively evaluated for impairment	67,432	59,373	57,100
Ending balance	75,782	87,396	84,181
Total loans:
Individually evaluated for impairment	123,406	210,199	162,804
Individually evaluated for impairment	2,660,420	2,544,495	2,709,125
Loans held for investment	2,783,826	2,754,694	2,871,929
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8,594	9,332	6,220
Provision charged to operating expense	1,922	3,566	8,636
Less loans charged-off	(5,320)	(6,043)	(7,577)
Add back recoveries of loans previously charged-off	1,945	1,739	2,053
Ending balance	7,141	8,594	9,332
Allowance for loan losses:
Loans individually evaluated for impairment	0	0	0
Loans collectively evaluated for impairment	7,141	8,594	9,332
Ending balance	7,141	8,594	9,332
Total loans:
Individually evaluated for impairment	0	0	0
Individually evaluated for impairment	636,794	616,071	646,580
Loans held for investment	636,794	616,071	646,580
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	15,325	25,354	18,608
Provision charged to operating expense	10,845	7,959	16,333
Less loans charged-off	(11,990)	(19,332)	(10,023)
Add back recoveries of loans previously charged-off	2,905	1,344	436
Ending balance	17,085	15,325	25,354
Allowance for loan losses:
Loans individually evaluated for impairment	1,919	4,664	14,892
Loans collectively evaluated for impairment	15,166	10,661	10,462
Ending balance	17,085	15,325	25,354
Total loans:
Individually evaluated for impairment	12,242	19,638	35,522
Individually evaluated for impairment	676,511	673,623	694,949
Loans held for investment	688,753	693,261	730,471
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,266	1,613	1,845
Provision charged to operating expense	(668)	(218)	(232)
Less loans charged-off	(120)	(142)	(21)
Add back recoveries of loans previously charged-off	25	13	21
Ending balance	503	1,266	1,613
Allowance for loan losses:
Loans individually evaluated for impairment	28	151	253
Loans collectively evaluated for impairment	475	1,115	1,360
Ending balance	503	1,266	1,613
Total loans:
Individually evaluated for impairment	537	759	976
Individually evaluated for impairment	113,090	118,951	115,570
Loans held for investment	113,627	119,710	116,546
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0	0	0
Provision charged to operating expense	0	0	0
Less loans charged-off	0	0	0
Add back recoveries of loans previously charged-off	0	0	0
Ending balance	0	0	0
Allowance for loan losses:
Loans individually evaluated for impairment	0	0	0
Loans collectively evaluated for impairment	0	0	0
Ending balance	0	0	0
Total loans:
Individually evaluated for impairment	0	0	0
Individually evaluated for impairment	912	2,813	2,383
Loans held for investment	$ 912	$ 2,813	$ 2,383