Schedule of Allowance for Loan Losses by Portfolio Segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 81,371	$ 97,904	$ 85,339	$ 100,511
Provision charged to operating expense	(2,001)	375	(7,001)	875
Less loans charged-off	(2,626)	(3,895)	(5,417)	(9,915)
Add back recoveries of loans previously charged-off	1,522	4,144	5,345	7,057
Ending balance	78,266	98,528	78,266	98,528
Allowance for loan losses:
Loans individually evaluated for impairment	6,952	15,494	6,952	15,494
Loans collectively evaluated for impairment	71,314	83,034	71,314	83,034
Ending balance	78,266	98,528	78,266	98,528
Total loans:
Individually evaluated for impairment	94,402	124,090	94,402	124,090
Individually evaluated for impairment	4,411,960	4,173,274	4,411,960	4,173,274
Total loans	4,506,362	4,297,364	4,506,362	4,297,364	4,344,853
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	59,830	71,666	63,923	75,782
Provision charged to operating expense	(2,011)	(1,009)	(5,386)	(2,044)
Less loans charged-off	(1,158)	(2,027)	(2,243)	(6,170)
Add back recoveries of loans previously charged-off	651	2,873	1,018	3,935
Ending balance	57,312	71,503	57,312	71,503
Allowance for loan losses:
Loans individually evaluated for impairment	5,555	10,267	5,555	10,267
Loans collectively evaluated for impairment	51,757	61,236	51,757	61,236
Ending balance	57,312	71,503	57,312	71,503
Total loans:
Individually evaluated for impairment	80,273	107,241	80,273	107,241
Individually evaluated for impairment	2,859,276	2,732,400	2,859,276	2,732,400
Total loans	2,939,549	2,839,641	2,939,549	2,839,641
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,377	7,088	6,193	7,140
Provision charged to operating expense	346	579	(232)	1,116
Less loans charged-off	(934)	(1,299)	(1,780)	(2,361)
Add back recoveries of loans previously charged-off	558	672	1,166	1,145
Ending balance	5,347	7,040	5,347	7,040
Allowance for loan losses:
Loans individually evaluated for impairment	0	0	0	0
Loans collectively evaluated for impairment	5,347	7,040	5,347	7,040
Ending balance	5,347	7,040	5,347	7,040
Total loans:
Individually evaluated for impairment	0	0	0	0
Individually evaluated for impairment	707,035	652,944	707,035	652,944
Total loans	707,035	652,944	707,035	652,944
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	15,701	18,670	14,747	17,085
Provision charged to operating expense	(494)	778	(1,566)	1,799
Less loans charged-off	(534)	(569)	(1,330)	(1,380)
Add back recoveries of loans previously charged-off	313	596	3,135	1,971
Ending balance	14,986	19,475	14,986	19,475
Allowance for loan losses:
Loans individually evaluated for impairment	1,176	5,205	1,176	5,205
Loans collectively evaluated for impairment	13,810	14,270	13,810	14,270
Ending balance	14,986	19,475	14,986	19,475
Total loans:
Individually evaluated for impairment	13,388	16,710	13,388	16,710
Individually evaluated for impairment	714,094	666,539	714,094	666,539
Total loans	727,482	683,249	727,482	683,249
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	463	480	476	504
Provision charged to operating expense	158	27	183	4
Less loans charged-off	0	0	(64)	(4)
Add back recoveries of loans previously charged-off	0	3	26	6
Ending balance	621	510	621	510
Allowance for loan losses:
Loans individually evaluated for impairment	221	22	221	22
Loans collectively evaluated for impairment	400	488	400	488
Ending balance	621	510	621	510
Total loans:
Individually evaluated for impairment	741	139	741	139
Individually evaluated for impairment	129,539	121,391	129,539	121,391
Total loans	130,280	121,530	130,280	121,530
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
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
Total loans:
Individually evaluated for impairment	0	0	0	0
Individually evaluated for impairment	2,016	0	2,016	0
Total loans	$ 2,016	$ 0	$ 2,016	$ 0