Loans and Allowance - Composition of Impaired Loans by Loan Class (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	$ 83,225		$ 83,225		$ 109,594
Impaired loans with related allowance - Unpaid Principal Balance	7,713		7,713		12,918
Unpaid Principal Balance	90,938		90,938		122,512
Impaired loans with no related allowance - Recorded Investment	47,603		47,603		67,333
Impaired loans with related allowance - Recorded Investment	6,624		6,624		11,846
Recorded Investment	54,227		54,227		79,179
Impaired loans with related allowance - Related Allowance	1,475		1,475		4,243
Impaired loans with no related allowance - Average Recorded Investment	49,875	55,699	52,494	61,126
Impaired loans with related allowance - Average Recorded Investment	6,681	19,565	6,811	19,704
Average Recorded Investment	56,556	75,264	59,305	80,830
Impaired loans with no related allowance - Interest Income Recognized	841	316	2,026	886
Impaired loans with related allowance - Interest Income Recognized	5	62	12	183
Interest Income Recognized	846	378	2,038	1,069
Commercial and industrial loans
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	21,683		21,683		28,532
Impaired loans with related allowance - Unpaid Principal Balance	1,119		1,119		4,415
Impaired loans with no related allowance - Recorded Investment	7,758		7,758		11,730
Impaired loans with related allowance - Recorded Investment	853		853		4,155
Impaired loans with related allowance - Related Allowance	401		401		1,628
Impaired loans with no related allowance - Average Recorded Investment	8,352	9,994	9,553	11,615
Impaired loans with related allowance - Average Recorded Investment	865	5,244	889	5,307
Impaired loans with no related allowance - Interest Income Recognized	110	26	184	68
Impaired loans with related allowance - Interest Income Recognized		11	7	32
Agriculture production financing and other loans to farmers
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	33		33
Impaired loans with no related allowance - Recorded Investment	31		31
Impaired loans with no related allowance - Average Recorded Investment	32	300	33	300
Impaired loans with no related allowance - Interest Income Recognized
Real estate loans - Construction
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	6,161		6,161		9,787
Impaired loans with related allowance - Unpaid Principal Balance					1,202
Impaired loans with no related allowance - Recorded Investment	3,789		3,789		5,164
Impaired loans with related allowance - Recorded Investment					1,058
Impaired loans with related allowance - Related Allowance					105
Impaired loans with no related allowance - Average Recorded Investment	4,115	6,658	4,262	7,665
Impaired loans with related allowance - Average Recorded Investment		510		541
Impaired loans with no related allowance - Interest Income Recognized	29	17	85	47
Impaired loans with related allowance - Interest Income Recognized
Real estate loans - Commercial and farm land
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	46,834		46,834		58,173
Impaired loans with related allowance - Unpaid Principal Balance	6,520		6,520		5,579
Impaired loans with no related allowance - Recorded Investment	33,085		33,085		43,204
Impaired loans with related allowance - Recorded Investment	5,700		5,700		5,182
Impaired loans with related allowance - Related Allowance	1,068		1,068		2,460
Impaired loans with no related allowance - Average Recorded Investment	34,298	31,081	35,198	33,459
Impaired loans with related allowance - Average Recorded Investment	5,742	12,326	5,844	12,371
Impaired loans with no related allowance - Interest Income Recognized	620	250	1,642	706
Impaired loans with related allowance - Interest Income Recognized	5	32	5	95
Real estate loans - Residential
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	4,671		4,671		8,820
Impaired loans with related allowance - Unpaid Principal Balance	74		74		1,722
Impaired loans with no related allowance - Recorded Investment	2,569		2,569		6,215
Impaired loans with related allowance - Recorded Investment	71		71		1,451
Impaired loans with related allowance - Related Allowance	6		6		50
Impaired loans with no related allowance - Average Recorded Investment	2,687	6,936	3,028	7,342
Impaired loans with related allowance - Average Recorded Investment	74	1,485	78	1,485
Impaired loans with no related allowance - Interest Income Recognized	82	20	114	54
Impaired loans with related allowance - Interest Income Recognized		19		56
Real estate loans - Home equity
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	3,386		3,386		4,199
Impaired loans with no related allowance - Recorded Investment	199		199		1,006
Impaired loans with no related allowance - Average Recorded Investment	217	713	236	727
Impaired loans with no related allowance - Interest Income Recognized		3		10
Other Loans
Financing Receivable, Impaired [Line Items]
Impaired loans with no related allowance - Unpaid Principal Balance	457		457		83
Impaired loans with related allowance - Unpaid Principal Balance
Impaired loans with no related allowance - Recorded Investment	172		172		14
Impaired loans with related allowance - Recorded Investment
Impaired loans with related allowance - Related Allowance
Impaired loans with no related allowance - Average Recorded Investment	174	17	184	18
Impaired loans with related allowance - Average Recorded Investment
Impaired loans with no related allowance - Interest Income Recognized				1
Impaired loans with related allowance - Interest Income Recognized