Fair Value Part 1 (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 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	$ 3,372,101	$ 2,657,745
Loans held for sale	119,986	257,986
Mortgage servicing rights	63,150	47,341	44,211	43,274
Other assets - derivatives	6,137	5,241
Other liabilities - derivatives	3,947	5,329
Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Treasury securities	503
U.S. Government agency obligations	265,438	105,745
Obligations of states and political subdivisions	212,991	215,761
Mortgage-backed securities	2,665,441	2,094,612
Asset-backed securities	227,728	241,627
Total, securities available for sale, estimated fair value	3,372,101	2,657,745
Loans held for sale	119,986	257,986
Mortgage servicing rights	63,150	47,341
Other assets - derivatives	10,841	7,107
Other liabilities - derivatives	7,305	6,612
Level 1 [Member] | Recurring Basis [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	2,633	(440)
Other liabilities - derivatives	480	545
Level 2 [Member] | Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Treasury securities	503
U.S. Government agency obligations	265,438	105,745
Obligations of states and political subdivisions	212,991	215,761
Mortgage-backed securities	2,665,441	2,094,612
Asset-backed securities	227,728	241,627
Total, securities available for sale, estimated fair value	3,372,101	2,657,745
Loans held for sale	119,986	257,986
Mortgage servicing rights	0	0
Other assets - derivatives	6,161	5,263
Other liabilities - derivatives	6,825	6,067
Level 3 [Member] | Recurring Basis [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	63,150	47,341
Other assets - derivatives	2,047	2,284
Other liabilities - derivatives	$ 0	$ 0