Fair Value of Assets and Liabilities (Fair value on recurring basis) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Assets:
Available-for-Sale	$ 2,188,064	$ 2,243,697
Loans Held for Sale	2,437	6,435
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Assets:
Available-for-Sale	61,832	70,693
Loans Held for Sale	0	0
Mortgage Servicing Rights	0	0
Other Assets	16,333	15,535
Derivatives	0	0
Total Assets Measured at Fair Value on a Recurring Basis	78,165	86,228
Liabilites:
Derivatives	0	0
Total Liabilities Measured at Fair Value on a Recurring Basis	0	0
Significant Other Observable Inputs (Level 2)
Assets:
Available-for-Sale	2,126,232	2,173,004
Loans Held for Sale	2,437	6,435
Mortgage Servicing Rights	0	0
Other Assets	0	0
Derivatives	78	343
Total Assets Measured at Fair Value on a Recurring Basis	2,128,747	2,179,782
Liabilites:
Derivatives	165	904
Total Liabilities Measured at Fair Value on a Recurring Basis	165	904
Significant Other Unobservable Inputs (Level 3)
Assets:
Available-for-Sale	0	0
Loans Held for Sale	0	0
Mortgage Servicing Rights	3,381	3,826
Other Assets	0	0
Derivatives	20,292	21,426
Total Assets Measured at Fair Value on a Recurring Basis	23,673	25,252
Liabilites:
Derivatives	20,003	21,047
Total Liabilities Measured at Fair Value on a Recurring Basis	20,003	21,047
Total
Assets:
Available-for-Sale	2,188,064	2,243,697
Loans Held for Sale	2,437	6,435
Mortgage Servicing Rights	3,381	3,826
Other Assets	16,333	15,535
Derivatives	20,370	21,769
Total Assets Measured at Fair Value on a Recurring Basis	2,230,585	2,291,262
Liabilites:
Derivatives	20,168	21,951
Total Liabilities Measured at Fair Value on a Recurring Basis	20,168	21,951
Debt Securities Issued by the U.S. Treasury and Government Agencies | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Assets:
Available-for-Sale	61,832	70,693
Debt Securities Issued by the U.S. Treasury and Government Agencies | Significant Other Observable Inputs (Level 2)
Assets:
Available-for-Sale	312,147	326,586
Debt Securities Issued by the U.S. Treasury and Government Agencies | Significant Other Unobservable Inputs (Level 3)
Assets:
Available-for-Sale	0	0
Debt Securities Issued by the U.S. Treasury and Government Agencies | Total
Assets:
Available-for-Sale	373,979	397,279
Debt Securities Issued by States and Political Subdivisions
Assets:
Available-for-Sale	723,119	686,802
Debt Securities Issued by States and Political Subdivisions | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Assets:
Available-for-Sale	0	0
Debt Securities Issued by States and Political Subdivisions | Significant Other Observable Inputs (Level 2)
Assets:
Available-for-Sale	723,119	686,802
Debt Securities Issued by States and Political Subdivisions | Significant Other Unobservable Inputs (Level 3)
Assets:
Available-for-Sale	0	0
Debt Securities Issued by States and Political Subdivisions | Total
Assets:
Available-for-Sale	723,119	686,802
Debt Securities Issued by Corporations
Assets:
Available-for-Sale	259,561	273,501
Debt Securities Issued by Corporations | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Assets:
Available-for-Sale	0	0
Debt Securities Issued by Corporations | Significant Other Observable Inputs (Level 2)
Assets:
Available-for-Sale	259,561	273,501
Debt Securities Issued by Corporations | Significant Other Unobservable Inputs (Level 3)
Assets:
Available-for-Sale	0	0
Debt Securities Issued by Corporations | Total
Assets:
Available-for-Sale	259,561	273,501
Mortgage-Backed Securities
Assets:
Available-for-Sale	831,405	886,115
Mortgage-Backed Securities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Assets:
Available-for-Sale	0	0
Mortgage-Backed Securities | Significant Other Observable Inputs (Level 2)
Assets:
Available-for-Sale	831,405	886,115
Mortgage-Backed Securities | Significant Other Unobservable Inputs (Level 3)
Assets:
Available-for-Sale	0	0
Mortgage-Backed Securities | Total
Assets:
Available-for-Sale	831,405	886,115
Residential Mortgage-Backed Securities issued by U.S. government agencies
Assets:
Available-for-Sale	604,983	653,194
Residential Mortgage-Backed Securities issued by U.S. government agencies | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Assets:
Available-for-Sale	0	0
Residential Mortgage-Backed Securities issued by U.S. government agencies | Significant Other Observable Inputs (Level 2)
Assets:
Available-for-Sale	604,983	653,194
Residential Mortgage-Backed Securities issued by U.S. government agencies | Significant Other Unobservable Inputs (Level 3)
Assets:
Available-for-Sale	0	0
Residential Mortgage-Backed Securities issued by U.S. government agencies | Total
Assets:
Available-for-Sale	604,983	653,194
Residential Mortgage-Backed Securities Issued by U.S. Government Sponsored Enterprises
Assets:
Available-for-Sale	21,408	23,268
Residential Mortgage-Backed Securities Issued by U.S. Government Sponsored Enterprises | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Assets:
Available-for-Sale	0	0
Residential Mortgage-Backed Securities Issued by U.S. Government Sponsored Enterprises | Significant Other Observable Inputs (Level 2)
Assets:
Available-for-Sale	21,408	23,268
Residential Mortgage-Backed Securities Issued by U.S. Government Sponsored Enterprises | Significant Other Unobservable Inputs (Level 3)
Assets:
Available-for-Sale	0	0
Residential Mortgage-Backed Securities Issued by U.S. Government Sponsored Enterprises | Total
Assets:
Available-for-Sale	21,408	23,268
Commercial Mortgage-Backed Securities Issued by Government Agencies
Assets:
Available-for-Sale	205,014	209,653
Commercial Mortgage-Backed Securities Issued by Government Agencies | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Assets:
Available-for-Sale	0	0
Commercial Mortgage-Backed Securities Issued by Government Agencies | Significant Other Observable Inputs (Level 2)
Assets:
Available-for-Sale	205,014	209,653
Commercial Mortgage-Backed Securities Issued by Government Agencies | Significant Other Unobservable Inputs (Level 3)
Assets:
Available-for-Sale	0	0
Commercial Mortgage-Backed Securities Issued by Government Agencies | Total
Assets:
Available-for-Sale	$ 205,014	$ 209,653