Loans and Leases Loans and leases not covered by share agreements (credit quality indicator) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	$ 2,634,249	$ 2,487,410
Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	2,342,352	2,197,563
Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	291,897	289,847
Commercial Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	1,903,996	1,809,450
Commercial Portfolio Segment | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	1,677,184	1,585,665
Commercial Portfolio Segment | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	226,812	223,785
Agricultural Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	279,285	262,975
Agricultural Portfolio Segment | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	243,092	223,247
Agricultural Portfolio Segment | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	36,193	39,728
Residential Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	220,084	194,436
Residential Portfolio Segment | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	201,274	177,128
Residential Portfolio Segment | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	18,810	17,308
Consumer Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	230,594	220,099
Consumer Portfolio Segment | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	220,512	211,073
Consumer Portfolio Segment | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	10,082	9,026
Finance Leases Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	290	450
Finance Leases Portfolio Segment | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	290	450
Finance Leases Portfolio Segment | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	0	0
Commercial | Commercial Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	667,251	645,666
Commercial | Commercial Portfolio Segment | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	620,320	596,759
Commercial | Commercial Portfolio Segment | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	46,931	48,907
Commercial Real Estate | Commercial Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	1,236,745	1,163,784
Commercial Real Estate | Commercial Portfolio Segment | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	1,056,864	988,906
Commercial Real Estate | Commercial Portfolio Segment | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	$ 179,881	$ 174,878