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Fair Value of Assets and Liabilities (Fair value on recurring basis) (Details 1) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets:
Available-for-Sale: Fair Value	$ 2,232,979	$ 2,186,041	$ 2,256,818
Loans Held for Sale	19,231	62,499
Derivative Assets	10,518	13,731
Fair Value, Measurements, Recurring [Member]
Assets:
Available-for-Sale: Fair Value	2,232,979	2,186,041
Loans Held for Sale	19,231	62,499
Mortgage Servicing Rights	1,454	1,655
Other Assets	29,230	21,952
Derivative Assets	10,518	13,731
Total Assets Measured at Fair Value on a Recurring Basis	2,293,412	2,285,878
Liabilities:
Derivative Liabilities	9,693	12,588
Total Liabilities Measured at Fair Value on a Recurring Basis	9,693	12,588
Fair Value, Measurements, Recurring [Member] \| Debt Securities Issued by the U.S. Treasury and Government Agencies
Assets:
Available-for-Sale: Fair Value	425,930	408,715
Fair Value, Measurements, Recurring [Member] \| Debt Securities Issued by States and Political Subdivisions
Assets:
Available-for-Sale: Fair Value	627,019	671,799
Fair Value, Measurements, Recurring [Member] \| Debt Securities Issued by Corporations
Assets:
Available-for-Sale: Fair Value	266,111	269,179
Fair Value, Measurements, Recurring [Member] \| Mortgage-Backed Securities
Assets:
Available-for-Sale: Fair Value	913,919	836,348
Fair Value, Measurements, Recurring [Member] \| Residential - Government Agencies
Assets:
Available-for-Sale: Fair Value	235,360	243,844
Fair Value, Measurements, Recurring [Member] \| Residential - U.S. Government Sponsored Enterprises
Assets:
Available-for-Sale: Fair Value	609,812	506,987
Fair Value, Measurements, Recurring [Member] \| Commercial - Government Agencies
Assets:
Available-for-Sale: Fair Value	68,747	85,517
Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Available-for-Sale: Fair Value	538	539
Loans Held for Sale	0	0
Mortgage Servicing Rights	0	0
Other Assets	29,230	21,952
Derivative Assets	0	0
Total Assets Measured at Fair Value on a Recurring Basis	29,768	22,491
Liabilities:
Derivative Liabilities	0	0
Total Liabilities Measured at Fair Value on a Recurring Basis	0	0
Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Debt Securities Issued by the U.S. Treasury and Government Agencies
Assets:
Available-for-Sale: Fair Value	538	539
Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Debt Securities Issued by States and Political Subdivisions
Assets:
Available-for-Sale: Fair Value	0	0
Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Debt Securities Issued by Corporations
Assets:
Available-for-Sale: Fair Value	0	0
Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Mortgage-Backed Securities
Assets:
Available-for-Sale: Fair Value	0	0
Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Residential - Government Agencies
Assets:
Available-for-Sale: Fair Value	0	0
Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Residential - U.S. Government Sponsored Enterprises
Assets:
Available-for-Sale: Fair Value	0	0
Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Commercial - Government Agencies
Assets:
Available-for-Sale: Fair Value	0	0
Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2)
Assets:
Available-for-Sale: Fair Value	2,232,441	2,185,502
Loans Held for Sale	19,231	62,499
Mortgage Servicing Rights	0	0
Other Assets	0	0
Derivative Assets	146	926
Total Assets Measured at Fair Value on a Recurring Basis	2,251,818	2,248,927
Liabilities:
Derivative Liabilities	215	836
Total Liabilities Measured at Fair Value on a Recurring Basis	215	836
Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2) \| Debt Securities Issued by the U.S. Treasury and Government Agencies
Assets:
Available-for-Sale: Fair Value	425,392	408,176
Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2) \| Debt Securities Issued by States and Political Subdivisions
Assets:
Available-for-Sale: Fair Value	627,019	671,799
Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2) \| Debt Securities Issued by Corporations
Assets:
Available-for-Sale: Fair Value	266,111	269,179
Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2) \| Mortgage-Backed Securities
Assets:
Available-for-Sale: Fair Value	913,919	836,348
Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2) \| Residential - Government Agencies
Assets:
Available-for-Sale: Fair Value	235,360	243,844
Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2) \| Residential - U.S. Government Sponsored Enterprises
Assets:
Available-for-Sale: Fair Value	609,812	506,987
Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2) \| Commercial - Government Agencies
Assets:
Available-for-Sale: Fair Value	68,747	85,517
Fair Value, Measurements, Recurring [Member] \| Significant Other Unobservable Inputs (Level 3)
Assets:
Available-for-Sale: Fair Value	0	0
Loans Held for Sale	0	0
Mortgage Servicing Rights	1,454	1,655
Other Assets	0	0
Derivative Assets	10,372	12,805
Total Assets Measured at Fair Value on a Recurring Basis	11,826	14,460
Liabilities:
Derivative Liabilities	9,478	11,752
Total Liabilities Measured at Fair Value on a Recurring Basis	9,478	11,752
Fair Value, Measurements, Recurring [Member] \| Significant Other Unobservable Inputs (Level 3) \| Debt Securities Issued by the U.S. Treasury and Government Agencies
Assets:
Available-for-Sale: Fair Value	0	0
Fair Value, Measurements, Recurring [Member] \| Significant Other Unobservable Inputs (Level 3) \| Debt Securities Issued by States and Political Subdivisions
Assets:
Available-for-Sale: Fair Value	0	0
Fair Value, Measurements, Recurring [Member] \| Significant Other Unobservable Inputs (Level 3) \| Debt Securities Issued by Corporations
Assets:
Available-for-Sale: Fair Value	0	0
Fair Value, Measurements, Recurring [Member] \| Significant Other Unobservable Inputs (Level 3) \| Mortgage-Backed Securities
Assets:
Available-for-Sale: Fair Value	0	0
Fair Value, Measurements, Recurring [Member] \| Significant Other Unobservable Inputs (Level 3) \| Residential - Government Agencies
Assets:
Available-for-Sale: Fair Value	0	0
Fair Value, Measurements, Recurring [Member] \| Significant Other Unobservable Inputs (Level 3) \| Residential - U.S. Government Sponsored Enterprises
Assets:
Available-for-Sale: Fair Value	0	0
Fair Value, Measurements, Recurring [Member] \| Significant Other Unobservable Inputs (Level 3) \| Commercial - Government Agencies
Assets:
Available-for-Sale: Fair Value	$ 0	$ 0