Loans and Leases (Loans and leases not covered by share agreements (credit quality indicator)) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	$ 2,792,873	$ 2,825,170
Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	2,514,613	2,545,323
Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	278,260	279,847
Commercial Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	1,990,818	2,001,492
Commercial Portfolio Segment | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	1,784,314	1,795,902
Commercial Portfolio Segment | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	206,504	205,590
Agricultural Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	314,606	328,311
Agricultural Portfolio Segment | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	271,019	286,264
Agricultural Portfolio Segment | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	43,587	42,047
Residential Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	240,453	249,689
Residential Portfolio Segment | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	222,487	227,925
Residential Portfolio Segment | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	17,966	21,764
Consumer Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	246,996	245,678
Consumer Portfolio Segment | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	236,793	235,232
Consumer Portfolio Segment | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	10,203	10,446
Commercial | Commercial Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	693,235	712,308
Commercial | Commercial Portfolio Segment | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	644,980	661,118
Commercial | Commercial Portfolio Segment | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	48,255	51,190
Commercial Real Estate | Commercial Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	1,297,583	1,289,184
Commercial Real Estate | Commercial Portfolio Segment | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	1,139,334	1,134,784
Commercial Real Estate | Commercial Portfolio Segment | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable	$ 158,249	$ 154,400