Fair Value Part 1 (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total, Securities Available for Sale, Estimated Fair Value	$ 3,546,083	$ 2,657,745
Loans held for sale	207,758	257,986
Mortgage servicing rights	51,529	47,341	45,893	43,274
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Treasury securities	506
U.S. Government agency obligations	327,519	105,745
Obligations of states and political subdivisions	218,467	215,761
Mortgage-backed securities	2,760,089	2,094,612
Asset-backed securities	239,502	241,627
Total, Securities Available for Sale, Estimated Fair Value	3,546,083	2,657,745
Loans held for sale	207,758	257,986
Mortgage servicing rights	51,529	47,341
Other assets - derivatives	7,223	7,107
Other liabilities - derivatives	6,242	6,612
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. Treasury securities	0
U.S. Government agency obligations	0	0
Obligations of states and political subdivisions	0	0
Mortgage-backed securities	0	0
Asset-backed securities	0	0
Total, Securities Available for Sale, Estimated Fair Value	0	0
Loans held for sale	0	0
Mortgage servicing rights	0	0
Other assets - derivatives	459	(440)
Other liabilities - derivatives	518	545
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. Treasury securities	506
U.S. Government agency obligations	327,519	105,745
Obligations of states and political subdivisions	218,467	215,761
Mortgage-backed securities	2,760,089	2,094,612
Asset-backed securities	239,502	241,627
Total, Securities Available for Sale, Estimated Fair Value	3,546,083	2,657,745
Loans held for sale	207,758	257,986
Mortgage servicing rights	0	0
Other assets - derivatives	4,827	5,263
Other liabilities - derivatives	5,724	6,067
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. Treasury securities	0
U.S. Government agency obligations	0	0
Obligations of states and political subdivisions	0	0
Mortgage-backed securities	0	0
Asset-backed securities	0	0
Total, Securities Available for Sale, Estimated Fair Value	0	0
Loans held for sale	0	0
Mortgage servicing rights	51,529	47,341
Other assets - derivatives	1,937	2,284
Other liabilities - derivatives	$ 0	$ 0