Schedule of Allowance for Loan Losses by Portfolio Segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance			$ 112,581	$ 120,480	$ 102,794		$ 124,579
Provision charged to operating expense	9,500	14,000	32,750	44,400
Less loans charged-off	(14,813)	(20,405)	(50,645)	(48,846)
Add back recoveries of loans previously charged-off	1,525	2,129	4,320	4,269
Ending balance	99,006	120,303	99,006	120,303	102,794		124,579
Allowance for loan losses:
Loans individually evaluated for impairment	14,684	37,005	14,684	37,005
Loans collectively evaluated for impairment	84,322	83,298	84,322	83,298
Ending balance	99,006	120,303	99,006	120,303	102,794		124,579
Total loans:
Individually evaluated for impairment	154,458	255,736	154,458	255,736
Individually evaluated for impairment	4,025,593	4,019,981	4,025,593	4,019,981
Total loans	4,180,051	4,275,717	4,180,051	4,275,717		4,186,549
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance			87,396	84,181	77,723		90,243
Provision charged to operating expense	5,371	9,984	22,172	33,080
Less loans charged-off	(9,138)	(12,210)	(36,645)	(29,598)
Add back recoveries of loans previously charged-off	387	1,386	1,420	1,740
Ending balance	74,343	89,403	74,343	89,403	77,723		90,243
Allowance for loan losses:
Loans individually evaluated for impairment	11,348	27,344	11,348	27,344
Loans collectively evaluated for impairment	62,995	62,059	62,995	62,059
Ending balance	74,343	89,403	74,343	89,403	77,723		90,243
Total loans:
Individually evaluated for impairment	140,460	223,386	140,460	223,386
Individually evaluated for impairment	2,600,908	2,582,202	2,600,908	2,582,202
Total loans	2,741,368	2,805,588	2,741,368	2,805,588
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance			8,594	9,332	7,274		8,526
Provision charged to operating expense	192	1,676	385	2,927
Less loans charged-off	(1,340)	(1,682)	(3,862)	(4,641)
Add back recoveries of loans previously charged-off	465	453	1,474	1,355
Ending balance	6,591	8,973	6,591	8,973	7,274		8,526
Allowance for loan losses:
Loans individually evaluated for impairment	0	0	0	0
Loans collectively evaluated for impairment	6,591	8,973	6,591	8,973
Ending balance	6,591	8,973	6,591	8,973	7,274		8,526
Total loans:
Individually evaluated for impairment	0	0	0	0
Individually evaluated for impairment	629,757	627,139	629,757	627,139
Total loans	629,757	627,139	629,757	627,139
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance			15,325	25,354	16,675		24,434
Provision charged to operating expense	4,222	2,206	10,530	8,461
Less loans charged-off	(4,335)	(6,498)	(10,028)	(14,547)
Add back recoveries of loans previously charged-off	666	287	1,401	1,161
Ending balance	17,228	20,429	17,228	20,429	16,675		24,434
Allowance for loan losses:
Loans individually evaluated for impairment	2,924	9,422	2,924	9,422
Loans collectively evaluated for impairment	14,304	11,007	14,304	11,007
Ending balance	17,228	20,429	17,228	20,429	16,675		24,434
Total loans:
Individually evaluated for impairment	13,408	31,470	13,408	31,470
Individually evaluated for impairment	658,692	671,540	658,692	671,540
Total loans	672,100	703,010	672,100	703,010
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance			1,266	1,613	1,122		1,376
Provision charged to operating expense	(285)	134	(337)	(68)
Less loans charged-off	0	(15)	(110)	(60)
Add back recoveries of loans previously charged-off	7	3	25	13
Ending balance	844	1,498	844	1,498	1,122		1,376
Allowance for loan losses:
Loans individually evaluated for impairment	412	239	412	239
Loans collectively evaluated for impairment	432	1,259	432	1,259
Ending balance	844	1,498	844	1,498	1,122		1,376
Total loans:
Individually evaluated for impairment	590	880	590	880
Individually evaluated for impairment	134,877	135,848	134,877	135,848
Total loans	135,467	136,728	135,467	136,728
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance			0	0	0		0
Provision charged to operating expense	0	0	0	0
Less loans charged-off	0	0	0	0
Add back recoveries of loans previously charged-off	0	0	0	0
Ending balance	0	0	0	0	0		0
Allowance for loan losses:
Loans individually evaluated for impairment	0	0	0	0
Loans collectively evaluated for impairment	0	0	0	0
Ending balance	0	0	0	0	0		0
Total loans:
Individually evaluated for impairment	0	0	0	0
Individually evaluated for impairment	1,359	3,252	1,359	3,252
Total loans	$ 1,359	$ 3,252	$ 1,359	$ 3,252