Investment Securities (Details 3) (USD $)	Mar. 31, 2013 security	Dec. 31, 2012
Investment securities in an unrealized loss position
Available-for-Sale, Less Than 12 Months, Fair Value	$ 816,925,000	$ 343,903,000
Available-for-Sale, Less Than 12 Months, Gross Unrealized Losses	(7,843,000)	(2,223,000)
Available-for-Sale, 12 Months or Longer, Fair Value	18,213,000	22,953,000
Available-for-Sale, 12 Months or Longer, Gross Unrealized Losses	(351,000)	(238,000)
Available-for-Sale, Total Fair Value	835,138,000	366,856,000
Available-for-Sale, Total Gross Unrealized Losses	(8,194,000)	(2,461,000)
Held-to-Maturity, Less Than 12 Months, Fair Value	446,953,000	351,762,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(2,379,000)	(1,366,000)
Held-to-Maturity, 12 Months or Longer, Fair Value	11,430,000
Held-to-maturity, 12 Months or Longer, Gross Unrealized Losses	(22,000)
Held-to-Maturity, Total Fair Value	458,383,000	351,762,000
Held-to-Maturity, Total Gross Unrealized Losses	(2,401,000)	(1,366,000)
Number of investment securities that were in an unrealized loss position	85
Carrying value of Federal Home Loan Bank stock	59,700,000
Carrying value of federal Reserve Bank stock	19,000,000
Debt Securities Issued by the U.S. Treasury and Government Agencies
Investment securities in an unrealized loss position
Available-for-Sale, Less Than 12 Months, Fair Value	2,205,000	2,295,000
Available-for-Sale, Less Than 12 Months, Gross Unrealized Losses	(12,000)	(14,000)
Available-for-Sale, 12 Months or Longer, Fair Value	490,000	564,000
Available-for-Sale, 12 Months or Longer, Gross Unrealized Losses	(3,000)	(3,000)
Available-for-Sale, Total Fair Value	2,695,000	2,859,000
Available-for-Sale, Total Gross Unrealized Losses	(15,000)	(17,000)
Debt Securities Issued by States and Political Subdivisions
Investment securities in an unrealized loss position
Available-for-Sale, Less Than 12 Months, Fair Value	244,042,000	72,400,000
Available-for-Sale, Less Than 12 Months, Gross Unrealized Losses	(2,227,000)	(955,000)
Available-for-Sale, 12 Months or Longer, Fair Value	100,000
Available-for-Sale, Total Fair Value	244,142,000	72,400,000
Available-for-Sale, Total Gross Unrealized Losses	(2,227,000)	(955,000)
Debt Securities Issued by Corporations
Investment securities in an unrealized loss position
Available-for-Sale, Less Than 12 Months, Fair Value	104,234,000
Available-for-Sale, Less Than 12 Months, Gross Unrealized Losses	(834,000)
Available-for-Sale, Total Fair Value	104,234,000
Available-for-Sale, Total Gross Unrealized Losses	(834,000)
Held-to-Maturity, Less Than 12 Months, Fair Value	23,372,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(172,000)
Held-to-Maturity, Total Fair Value	23,372,000
Held-to-Maturity, Total Gross Unrealized Losses	(172,000)
Mortgage-Backed Securities
Investment securities in an unrealized loss position
Available-for-Sale, Less Than 12 Months, Fair Value	466,444,000	269,208,000
Available-for-Sale, Less Than 12 Months, Gross Unrealized Losses	(4,770,000)	(1,254,000)
Available-for-Sale, 12 Months or Longer, Fair Value	17,623,000	22,389,000
Available-for-Sale, 12 Months or Longer, Gross Unrealized Losses	(348,000)	(235,000)
Available-for-Sale, Total Fair Value	484,067,000	291,597,000
Available-for-Sale, Total Gross Unrealized Losses	(5,118,000)	(1,489,000)
Residential Mortgage-Backed Securities issued by U.S. government agencies
Investment securities in an unrealized loss position
Available-for-Sale, Less Than 12 Months, Fair Value	11,788,000	7,325,000
Available-for-Sale, Less Than 12 Months, Gross Unrealized Losses	(46,000)	(57,000)
Available-for-Sale, 12 Months or Longer, Fair Value	17,623,000	22,389,000
Available-for-Sale, 12 Months or Longer, Gross Unrealized Losses	(348,000)	(235,000)
Available-for-Sale, Total Fair Value	29,411,000	29,714,000
Available-for-Sale, Total Gross Unrealized Losses	(394,000)	(292,000)
Held-to-Maturity, Less Than 12 Months, Fair Value	423,581,000	351,762,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(2,207,000)	(1,366,000)
Held-to-Maturity, 12 Months or Longer, Fair Value	11,430,000
Held-to-maturity, 12 Months or Longer, Gross Unrealized Losses	(22,000)
Held-to-Maturity, Total Fair Value	435,011,000	351,762,000
Held-to-Maturity, Total Gross Unrealized Losses	(2,229,000)	(1,366,000)
Commercial Mortgage-Backed Securities Issued by Government Agencies
Investment securities in an unrealized loss position
Available-for-Sale, Less Than 12 Months, Fair Value	454,656,000	261,883,000
Available-for-Sale, Less Than 12 Months, Gross Unrealized Losses	(4,724,000)	(1,197,000)
Available-for-Sale, Total Fair Value	454,656,000	261,883,000
Available-for-Sale, Total Gross Unrealized Losses	$ (4,724,000)	$ (1,197,000)