Loans and Allowance - Composition of Impaired Loans by Loan Class (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
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	$ 164,599		$ 164,599		$ 189,779
Impaired loans with related allowance - Unpaid Principal Balance	11,848		11,848		6,121
Unpaid Principal Balance	176,447		176,447		195,900
Impaired loans with no related allowance - Recorded Investment	103,561		103,561		114,253
Impaired loans with related allowance - Recorded Investment	8,650		8,650		5,502
Recorded Investment	112,211		112,211		119,755
Impaired loans with related allowance - Related Allowance	1,708		1,708		1,583
Impaired loans with no related allowance - Average Recorded Investment	104,349	48,213	106,011	49,082
Impaired loans with related allowance - Average Recorded Investment	8,712	15,885	8,794	16,456
Average Recorded Investment	113,061	64,098	114,805	65,538
Impaired loans with no related allowance - Interest Income Recognized	1,199	454	2,425	904
Impaired loans with related allowance - Interest Income Recognized	15	3	30	5
Interest Income Recognized	1,214	457	2,455	909
Commercial and industrial loans
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	29,755		29,755		35,066
Impaired loans with related allowance - Unpaid Principal Balance	5,107		5,107		1,390
Impaired loans with no related allowance - Recorded Investment	11,978		11,978		16,371
Impaired loans with related allowance - Recorded Investment	3,573		3,573		1,216
Impaired loans with related allowance - Related Allowance	942		942		683
Impaired loans with no related allowance - Average Recorded Investment	12,060	5,864	12,872	6,138
Impaired loans with related allowance - Average Recorded Investment	3,575	5,669	3,590	6,138
Impaired loans with no related allowance - Interest Income Recognized	84	35	185	69
Impaired loans with related allowance - Interest Income Recognized	10	3	20	5
Agricultural production financing and other loans to farmers
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	29		29		32
Impaired loans with no related allowance - Recorded Investment	26		26		30
Impaired loans with no related allowance - Average Recorded Investment	27	33	28	33
Real estate loans - Construction
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	15,119		15,119		16,109
Impaired loans with no related allowance - Recorded Investment	10,235		10,235		10,625
Impaired loans with no related allowance - Average Recorded Investment	10,331	3,060	10,412	3,070
Impaired loans with related allowance - Average Recorded Investment		599		599
Impaired loans with no related allowance - Interest Income Recognized	114	19	227	38
Real estate loans - Commercial and farm land
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	109,955		109,955		128,073
Impaired loans with related allowance - Unpaid Principal Balance	6,741		6,741		4,657
Impaired loans with no related allowance - Recorded Investment	77,211		77,211		83,033
Impaired loans with related allowance - Recorded Investment	5,077		5,077		4,215
Impaired loans with related allowance - Related Allowance	766		766		894
Impaired loans with no related allowance - Average Recorded Investment	77,716	32,932	78,288	33,192
Impaired loans with related allowance - Average Recorded Investment	5,137	9,227	5,204	9,323
Impaired loans with no related allowance - Interest Income Recognized	970	382	1,956	760
Impaired loans with related allowance - Interest Income Recognized	5		10
Real estate loans - Residential
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	6,317		6,317		6,746
Impaired loans with related allowance - Unpaid Principal Balance					74
Impaired loans with no related allowance - Recorded Investment	3,915		3,915		3,910
Impaired loans with related allowance - Recorded Investment					71
Impaired loans with related allowance - Related Allowance					6
Impaired loans with no related allowance - Average Recorded Investment	4,017	6,067	4,212	6,372
Impaired loans with related allowance - Average Recorded Investment		238		240
Impaired loans with no related allowance - Interest Income Recognized	31	18	57	37
Real estate loans - Home equity
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	3,389		3,389		3,299
Impaired loans with no related allowance - Recorded Investment	196		196		112
Impaired loans with no related allowance - Average Recorded Investment	198	226	199	245
Other loans
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	35		35		454
Impaired loans with no related allowance - Recorded Investment					172
Impaired loans with no related allowance - Average Recorded Investment		31		32
Impaired loans with related allowance - Average Recorded Investment		$ 152		$ 156