Fair Value of Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Assets:
Available-for-Sale	$ 2,277,136	$ 3,367,557
Loans Held for Sale	18,795	21,374
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Assets:
Available-for-Sale	110,917	470,535
Loans Held for Sale	0	0
Mortgage Servicing Rights	0	0
Other Assets	14,153	12,566
Derivatives	0	0
Total Assets Measured at Fair Value on a Recurring Basis	125,070	483,101
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	2,166,219	2,897,022
Loans Held for Sale	18,795	21,374
Mortgage Servicing Rights	0	0
Other Assets	0	0
Derivatives	189	229
Total Assets Measured at Fair Value on a Recurring Basis	2,185,203	2,918,625
Liabilites:
Derivatives	1,103	1,185
Total Liabilities Measured at Fair Value on a Recurring Basis	1,103	1,185
Significant Other Unobservable Inputs (Level 3)
Assets:
Available-for-Sale	0	0
Loans Held for Sale	0	0
Mortgage Servicing Rights	3,819	4,761
Other Assets	0	0
Derivatives	25,390	42,381
Total Assets Measured at Fair Value on a Recurring Basis	29,209	47,142
Liabilites:
Derivatives	23,348	32,441
Total Liabilities Measured at Fair Value on a Recurring Basis	23,348	32,441
Total
Assets:
Available-for-Sale	2,277,136	3,367,557
Loans Held for Sale	18,795	21,374
Mortgage Servicing Rights	3,819	4,761
Other Assets	14,153	12,566
Derivatives	25,579	42,610
Total Assets Measured at Fair Value on a Recurring Basis	2,339,482	3,448,868
Liabilites:
Derivatives	24,451	33,626
Total Liabilities Measured at Fair Value on a Recurring Basis	24,451	33,626
Debt Securities Issued by the U.S. Treasury and Government Agencies
Assets:
Available-for-Sale	455,821	869,989
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	110,917	470,535
Debt Securities Issued by the U.S. Treasury and Government Agencies | Significant Other Observable Inputs (Level 2)
Assets:
Available-for-Sale	344,904	399,454
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	455,821	869,989
Debt Securities Issued by States and Political Subdivisions
Assets:
Available-for-Sale	587,239	782,411
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	587,239	782,411
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	587,239	782,411
Debt Securities Issued by Corporations
Assets:
Available-for-Sale	275,256	84,434
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	275,256	84,434
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	275,256	84,434
Mortgage-Backed Securities
Assets:
Available-for-Sale	958,820	1,630,723
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	958,820	1,630,723
Mortgage-Backed Securities | Significant Other Unobservable Inputs (Level 3)
Assets:
Available-for-Sale	0	0
Mortgage-Backed Securities | Total
Assets:
Available-for-Sale	958,820	1,630,723
Residential Mortgage-Backed Securities issued by U.S. government agencies
Assets:
Available-for-Sale	717,474	1,068,660
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	717,474	1,068,660
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	717,474	1,068,660
Residential Mortgage-Backed Securities Issued by U.S. Government Sponsored Enterprises
Assets:
Available-for-Sale	25,263	37,298
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	25,263	37,298
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	25,263	37,298
Commercial Mortgage-Backed Securities Issued by Government Agencies
Assets:
Available-for-Sale	216,083	524,765
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	216,083	524,765
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	$ 216,083	$ 524,765