Fair Value (Quantitative Information About Level 3 Fair Value Measurements) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Securities available for sale	$ 1,633,902		$ 1,490,331
Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Securities available for sale	3,298		4,089
Other real estate owned	29,852	[1]	35,822	[1]
Mortgage servicing rights			15,956
Commercial | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	7,229	[1]	7,681	[1]
Commercial Real Estate | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	7,749	[1]	44,471	[1]
Agricultural and Agricultural Real Estate | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	13,062	[1]	16,019	[1]
Residential Real Estate | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	3,396	[1]	6,660	[1]
Consumer | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	1,763	[1]	4,121	[1]
Z-Tranche Securities | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Securities available for sale	3,298
Z-Tranche Securities | Weighted Average | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Actual defaults	20.80%		15.52%
Actual deferrals	11.10%		11.32%
Z-Tranche Securities | Minimum | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate	7.00%		8.00%
Actual defaults	12.50%		13.94%
Actual deferrals	5.10%		6.30%
Z-Tranche Securities | Maximum | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate	9.00%		10.00%
Actual defaults	28.20%		20.94%
Actual deferrals	16.00%		23.71%
Mortgage Servicing Rights | Weighted Average | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Actual deferrals	18.05%		18.05%
Mortgage Servicing Rights | Minimum | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Actual deferrals	12.99%		12.99%
Mortgage Servicing Rights | Maximum | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Actual deferrals	18.71%		18.71%
Nonrecurring Basis | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	33,199		78,952
Other real estate owned	29,852		35,822
Mortgage servicing rights			15,956
Nonrecurring Basis | Commercial | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	7,229		7,681
Nonrecurring Basis | Commercial Real Estate | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	7,749		44,471
Nonrecurring Basis | Agricultural and Agricultural Real Estate | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	13,062		16,019
Nonrecurring Basis | Residential Real Estate | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	3,396		6,660
Nonrecurring Basis | Consumer | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	1,763		4,121
Nonrecurring Basis | Z-Tranche Securities | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Securities available for sale			$ 4,089

[1] Not Meaningful. Third party appraisals are obtained as to the value of the underlying asset, but disclosure of this information would not provide meaningful information, as the range will vary widely from loan to loan. Types of discounts considered included age of the appraisal, local market conditions, current condition of the property, and estimated sales costs. These discounts will also vary from loan to loan, thus providing range would not be meaningful.