Investment Securities (Details 4) (USD $)	Jun. 30, 2011	Dec. 31, 2010
Investment securities in an unrealized loss position
Less Than 12 Months, Fair Value	$ 583,383,000	$ 1,732,017,000
Less Than 12 Months, Gross Unrealized Losses	(5,287,000)	(32,506,000)
12 Months or Longer, Fair Value	2,592,000	1,204,000
12 Months or Longer, Gross Unrealized Losses	(25,000)	(9,000)
Total Fair Value	585,975,000	1,733,221,000
Total Gross Unrealized Losses	(5,312,000)	(32,515,000)
Number of investment securities that were in an unrealized loss position	62
Carrying value of federal Home Loan Bank stock	61,300,000
Carrying value of federal Reserve Bank stock	18,600,000
Debt Securities Issued by the U.S. Treasury and Government Agencies
Investment securities in an unrealized loss position
Less Than 12 Months, Fair Value	69,999,000	1,366,000
Less Than 12 Months, Gross Unrealized Losses	(813,000)	(36,000)
12 Months or Longer, Fair Value	1,065,000	1,204,000
12 Months or Longer, Gross Unrealized Losses	(4,000)	(9,000)
Total Fair Value	71,064,000	2,570,000
Total Gross Unrealized Losses	(817,000)	(45,000)
Debt Securities Issued by States and Political Subdivisions
Investment securities in an unrealized loss position
Less Than 12 Months, Fair Value	28,745,000	67,754,000
Less Than 12 Months, Gross Unrealized Losses	(280,000)	(1,583,000)
Total Fair Value	28,745,000	67,754,000
Total Gross Unrealized Losses	(280,000)	(1,583,000)
Mortgage-Backed Securities Issued by Government Agencies
Investment securities in an unrealized loss position
Less Than 12 Months, Fair Value	479,590,000	1,662,897,000
Less Than 12 Months, Gross Unrealized Losses	(4,193,000)	(30,887,000)
12 Months or Longer, Fair Value	1,527,000
12 Months or Longer, Gross Unrealized Losses	(21,000)
Total Fair Value	481,117,000	1,662,897,000
Total Gross Unrealized Losses	(4,214,000)	(30,887,000)
Debt Securities Issued by Corporations
Investment securities in an unrealized loss position
Less Than 12 Months, Fair Value	5,049,000
Less Than 12 Months, Gross Unrealized Losses	(1,000)
Total Fair Value	5,049,000
Total Gross Unrealized Losses	$ (1,000)