Schedule of Allowance for Loan Losses by Portfolio Segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 97,904	$ 115,902	$ 100,511	$ 112,581
Provision charged to operating expense	375	12,000	875	23,250
Less loans charged-off	(3,895)	(26,745)	(9,915)	(35,832)
Add back recoveries of loans previously charged-off	4,144	1,637	7,057	2,795
Ending balance	98,528	102,794	98,528	102,794
Allowance for loan losses:
Loans individually evaluated for impairment	15,494		15,494
Loans collectively evaluated for impairment	83,034		83,034
Ending balance	98,528	102,794	98,528	102,794
Total loans:
Individually evaluated for impairment	124,090		124,090
Individually evaluated for impairment	4,173,274		4,173,274
Total loans	4,297,364	4,169,963	4,297,364	4,169,963	4,223,912
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	71,666	89,128	75,782	87,396
Provision charged to operating expense	(1,009)	10,419	(2,044)	16,801
Less loans charged-off	(2,027)	(22,350)	(6,170)	(27,506)
Add back recoveries of loans previously charged-off	2,873	526	3,935	1,032
Ending balance	71,503	77,723	71,503	77,723
Allowance for loan losses:
Loans individually evaluated for impairment	10,267		10,267
Loans collectively evaluated for impairment	61,236		61,236
Ending balance	71,503	77,723	71,503	77,723
Total loans:
Individually evaluated for impairment	107,241		107,241
Individually evaluated for impairment	2,732,400		2,732,400
Total loans	2,839,641	2,688,307	2,839,641	2,688,307
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	7,088	7,212	7,140	8,594
Provision charged to operating expense	579	784	1,116	193
Less loans charged-off	(1,299)	(1,210)	(2,361)	(2,522)
Add back recoveries of loans previously charged-off	672	488	1,145	1,009
Ending balance	7,040	7,274	7,040	7,274
Allowance for loan losses:
Loans individually evaluated for impairment	0		0
Loans collectively evaluated for impairment	7,040		7,040
Ending balance	7,040	7,274	7,040	7,274
Total loans:
Individually evaluated for impairment	0		0
Individually evaluated for impairment	652,944		652,944
Total loans	652,944	621,212	652,944	621,212
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	18,652	18,380	17,073	15,325
Provision charged to operating expense	763	866	1,778	6,307
Less loans charged-off	(569)	(3,180)	(1,380)	(5,692)
Add back recoveries of loans previously charged-off	596	609	1,971	735
Ending balance	19,442	16,675	19,442	16,675
Allowance for loan losses:
Loans individually evaluated for impairment	5,205		5,205
Loans collectively evaluated for impairment	14,237		14,237
Ending balance	19,442	16,675	19,442	16,675
Total loans:
Individually evaluated for impairment	16,710		16,710
Individually evaluated for impairment	664,041		664,041
Total loans	680,751	720,010	680,751	720,010
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	480	1,182	504	1,266
Provision charged to operating expense	27	(69)	4	(51)
Less loans charged-off	0	(5)	(4)	(112)
Add back recoveries of loans previously charged-off	3	14	6	19
Ending balance	510	1,122	510	1,122
Allowance for loan losses:
Loans individually evaluated for impairment	22		22
Loans collectively evaluated for impairment	488		488
Ending balance	510	1,122	510	1,122
Total loans:
Individually evaluated for impairment	139		139
Individually evaluated for impairment	121,391		121,391
Total loans	121,530	138,115	121,530	138,115
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	18	0	12	0
Provision charged to operating expense	15	0	21	0
Less loans charged-off	0	0	0	0
Add back recoveries of loans previously charged-off	0	0	0	0
Ending balance	33	0	33	0
Allowance for loan losses:
Loans individually evaluated for impairment	0		0
Loans collectively evaluated for impairment	33		33
Ending balance	33	0	33	0
Total loans:
Individually evaluated for impairment	0		0
Individually evaluated for impairment	2,498		2,498
Total loans	$ 2,498	$ 2,319	$ 2,498	$ 2,319