Fair Value Measurement (Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis) (Details) (USD $)	Jun. 30, 2011		Mar. 31, 2011	Dec. 31, 2010		Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2009
Trading securities	$ 2,522,000			$ 3,024,000
Available for sale securities	3,177,460,000			2,919,180,000
Mortgage servicing rights, at fair value	16,350,000.000		15,605,000.000	14,454,000.000		12,895,000.000	13,628,000.000	12,625,000.000
Derivatives	1,979,000			1,060,000
Total assets measured at fair value	3,198,311,000			2,937,718,000
Junior subordinated debentures, at fair value	81,766,000			80,688,000
Derivatives	1,518,000			361,000
Total liabilities measured at fair value	83,284,000			81,049,000
Obligations of States and Political Subdivisions [Member]
Trading securities	501,000			1,282,000
Available for sale securities	222,898,000			216,726,000
Obligations of States and Political Subdivisions [Member] | Level 1 [Member]
Trading securities	501,000			1,282,000
Obligations of States and Political Subdivisions [Member] | Level 2 [Member]
Available for sale securities	222,898,000			216,726,000
Trading Equity Securities [Member]
Trading securities	1,926,000			1,645,000
Trading Equity Securities [Member] | Level 1 [Member]
Trading securities	1,926,000			1,645,000
Other Investments Securities [Member]
Trading securities	95,000	[1]		97,000	[1]
Other Investments Securities [Member] | Level 1 [Member]
Trading securities	95,000	[1]		97,000	[1]
U.S. Treasury and Agencies [Member]
Available for sale securities	118,953,000			118,789,000
U.S. Treasury and Agencies [Member] | Level 2 [Member]
Available for sale securities	118,953,000			118,789,000
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations [Member]
Available for sale securities	2,833,429,000			2,581,504,000
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations [Member] | Level 2 [Member]
Available for sale securities	2,833,429,000			2,581,504,000
Other Debt Securities [Member]
Available for sale securities	150,000			152,000
Other Debt Securities [Member] | Level 2 [Member]
Available for sale securities	150,000			152,000
Investments in Mutual Funds and Other Equity Securities [Member]
Available for sale securities	2,030,000			2,009,000
Investments in Mutual Funds and Other Equity Securities [Member] | Level 2 [Member]
Available for sale securities	2,030,000			2,009,000
Level 1 [Member]
Total assets measured at fair value	2,522,000			3,024,000
Level 2 [Member]
Derivatives	1,979,000			1,060,000
Total assets measured at fair value	3,179,439,000			2,920,240,000
Derivatives	1,518,000			361,000
Total liabilities measured at fair value	1,518,000			361,000
Level 3 [Member]
Mortgage servicing rights, at fair value	16,350,000			14,454,000
Total assets measured at fair value	16,350,000			14,454,000
Junior subordinated debentures, at fair value	81,766,000			80,688,000
Total liabilities measured at fair value	$ 81,766,000			$ 80,688,000

[1] Principally represents U.S. Treasury and agencies or residential mortgage-backed securities issued or guaranteed by governmental agencies