Fair Value Part 1 (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	$ 2,592,807	$ 2,468,993
Loans held for sale	286,221	216,553
Mortgage servicing rights	43,580	43,274	50,111	51,151
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Government agency obligations	72,945	64,805
Obligations of states and political subdivisions	213,826	202,827
Mortgage-backed securities	2,211,064	2,201,361
Asset-backed securities	91,293
Corporate debt securities	3,679
Securities available for sale	2,592,807	2,468,993
Loans held for sale	286,221	216,553
Mortgage servicing rights	43,580	43,274
Other assets - derivatives	6,973	3,521
Other liabilities - derivatives	8,488	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	399	1,130
Other liabilities - derivatives	791	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	72,945	64,805
Obligations of states and political subdivisions	213,826	202,827
Mortgage-backed securities	2,211,064	2,201,361
Asset-backed securities	91,293
Corporate debt securities	3,679
Securities available for sale	2,592,807	2,468,993
Loans held for sale	286,221	216,553
Mortgage servicing rights	0	0
Other assets - derivatives	4,239	1,689
Other liabilities - derivatives	7,697	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	43,580	43,274
Other assets - derivatives	2,335	702
Other liabilities - derivatives	$ 0	$ 0