Fair Value Part 1 (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Government agency obligations	$ 101,972	$ 64,805
Obligations of states and political subdivisions	208,234	202,827
Mortgage-backed securities	2,256,471	2,201,361
Asset-backed securities	23,693
Corporate debt securities	5,294
Securities available for sale	2,595,664	2,468,993
Loans held for sale	227,449	216,553
Mortgage servicing rights	45,893	43,274	53,598	51,151
Other assets - derivatives	1,504	3,521
Other liabilities - derivatives	1,842	4,680
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Government agency obligations	0	0
Obligations of states and political subdivisions	0	0
Mortgage-backed securities	0	0
Asset-backed securities	0
Corporate debt securities	0
Securities available for sale	0	0
Loans held for sale	0	0
Mortgage servicing rights	0	0
Other assets - derivatives	(611)	1,130
Other liabilities - derivatives	361	694
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Government agency obligations	101,972	64,805
Obligations of states and political subdivisions	208,234	202,827
Mortgage-backed securities	2,256,471	2,201,361
Asset-backed securities	23,693
Corporate debt securities	5,294
Securities available for sale	2,595,664	2,468,993
Loans held for sale	227,449	216,553
Mortgage servicing rights	0	0
Other assets - derivatives	1,613	1,689
Other liabilities - derivatives	1,481	3,986
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Government agency obligations	0	0
Obligations of states and political subdivisions	0	0
Mortgage-backed securities	0	0
Asset-backed securities	0
Corporate debt securities	0
Securities available for sale	0	0
Loans held for sale	0	0
Mortgage servicing rights	45,893	43,274
Other assets - derivatives	502	702
Other liabilities - derivatives	$ 0	$ 0