Fair Value of Financial Instruments - Fair value of balance sheet items (Details) - USD ($) $ in Thousands	Mar. 31, 2017		Dec. 31, 2016
Assets:
Trading account assets	$ 2,892,254		$ 3,144,600
Investment securities available for sale	11,846,618		11,665,055
Loans held for sale, estimated fair value	74,741		105,257
Derivative asset	365,882		425,811
Other assets	1,532,837		1,435,187
Liabilities:
Derivative liabilities	325,035		345,334
U.S. Treasury and other U.S. government agencies
Assets:
Investment securities available for sale	2,967,385		2,374,331
Collateralized mortgage obligations
Assets:
Investment securities available for sale	4,934,143		5,098,928
State and political subdivisions
Assets:
Investment securities available for sale	8,208		8,641
Other debt securities
Assets:
Investment securities available for sale	14,061		16,185
Equity Securities
Assets:
Investment securities available for sale	400,797		403,632
Fair Value, measurements, recurring
Assets:
Trading account assets	2,892,254		3,144,600
Investment securities available for sale	11,446,191		11,261,803
Loans held for sale, estimated fair value	74,741		105,257
Derivative asset	97,724		107,206
Liabilities:
Trading account liabilities	2,807,357		3,008,042
Derivative liabilities	108,272		88,561
Fair Value, measurements, recurring | Interest rate contracts
Assets:
Derivative asset	35,559		46,133
Liabilities:
Derivative liabilities	48,925		33,414
Fair Value, measurements, recurring | Equity contracts
Assets:
Derivative asset	60,829		57,198
Liabilities:
Derivative liabilities	56,178		53,044
Fair Value, measurements, recurring | Foreign exchange contracts
Assets:
Derivative asset	1,336		3,875
Liabilities:
Derivative liabilities	3,169		2,103
Fair Value, measurements, recurring | U.S. Treasury and other U.S. government agencies
Assets:
Trading account assets	2,614,008		2,820,797
Investment securities available for sale	2,967,385		2,374,331
Liabilities:
Trading account liabilities	2,584,514		2,750,085
Fair Value, measurements, recurring | Collateralized mortgage obligations
Assets:
Trading account assets	1,324
Investment securities available for sale	4,934,143		5,098,928
Fair Value, measurements, recurring | State and political subdivisions
Assets:
Trading account assets	460		219
Investment securities available for sale	8,208		8,641
Fair Value, measurements, recurring | Other debt securities
Assets:
Trading account assets	7,485		4,120
Investment securities available for sale	14,061		16,185
Liabilities:
Trading account liabilities	8,025		2,892
Fair Value, measurements, recurring | Interest rate contracts
Assets:
Trading account assets	244,525		290,238
Liabilities:
Trading account liabilities	193,258		228,748
Fair Value, measurements, recurring | Foreign exchange contracts
Assets:
Trading account assets	23,633		28,367
Liabilities:
Trading account liabilities	21,560		26,317
Fair Value, measurements, recurring | Other trading assets
Assets:
Trading account assets	819		859
Fair Value, measurements, recurring | Mortgage-backed securities
Assets:
Investment securities available for sale	3,522,024		3,763,338
Fair Value, measurements, recurring | Equity Securities
Assets:
Investment securities available for sale	370	[1]	380	[2]
Fair Value, measurements, recurring | Quoted Prices in Active Markets for Identical Assets, Level 1
Assets:
Trading account assets	2,614,008		2,820,797
Investment securities available for sale	1,897,425		1,282,836
Liabilities:
Trading account liabilities	2,581,472		2,750,085
Fair Value, measurements, recurring | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. Treasury and other U.S. government agencies
Assets:
Trading account assets	2,614,008		2,820,797
Investment securities available for sale	1,883,291		1,266,564
Liabilities:
Trading account liabilities	2,581,472		2,750,085
Fair Value, measurements, recurring | Quoted Prices in Active Markets for Identical Assets, Level 1 | Other debt securities
Assets:
Investment securities available for sale	14,061		16,185
Fair Value, measurements, recurring | Quoted Prices in Active Markets for Identical Assets, Level 1 | Equity Securities
Assets:
Investment securities available for sale	73	[1]	87	[2]
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2
Assets:
Trading account assets	277,427		322,944
Investment securities available for sale	9,548,469		9,978,674
Loans held for sale, estimated fair value	74,741		105,257
Derivative asset	94,546		104,782
Liabilities:
Trading account liabilities	225,885		257,957
Derivative liabilities	108,271		88,529
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Interest rate contracts
Assets:
Derivative asset	32,381		43,709
Liabilities:
Derivative liabilities	48,924		33,382
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Equity contracts
Assets:
Derivative asset	60,829		57,198
Liabilities:
Derivative liabilities	56,178		53,044
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Foreign exchange contracts
Assets:
Derivative asset	1,336		3,875
Liabilities:
Derivative liabilities	3,169		2,103
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | U.S. Treasury and other U.S. government agencies
Assets:
Investment securities available for sale	1,084,094		1,107,767
Liabilities:
Trading account liabilities	3,042
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Collateralized mortgage obligations
Assets:
Trading account assets	1,324
Investment securities available for sale	4,934,143		5,098,928
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | State and political subdivisions
Assets:
Trading account assets	460		219
Investment securities available for sale	8,208		8,641
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Other debt securities
Assets:
Trading account assets	7,485		4,120
Liabilities:
Trading account liabilities	8,025		2,892
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Interest rate contracts
Assets:
Trading account assets	244,525		290,238
Liabilities:
Trading account liabilities	193,258		228,748
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Foreign exchange contracts
Assets:
Trading account assets	23,633		28,367
Liabilities:
Trading account liabilities	21,560		26,317
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Mortgage-backed securities
Assets:
Investment securities available for sale	3,522,024		3,763,338
Fair Value, measurements, recurring | Fair Value, Inputs, Level 3
Assets:
Trading account assets	819		859
Investment securities available for sale	297		293
Derivative asset	3,178		2,424
Liabilities:
Derivative liabilities	1		32
Fair Value, measurements, recurring | Fair Value, Inputs, Level 3 | Interest rate contracts
Assets:
Derivative asset	3,178		2,424
Liabilities:
Derivative liabilities	1		32
Fair Value, measurements, recurring | Fair Value, Inputs, Level 3 | Other trading assets
Assets:
Trading account assets	819		859
Fair Value, measurements, recurring | Fair Value, Inputs, Level 3 | Equity Securities
Assets:
Investment securities available for sale	297	[1]	293	[2]
Other assets, MSR [Member] | Fair Value, measurements, recurring
Assets:
Other assets	50,776		51,428
Other assets, MSR [Member] | Fair Value, measurements, recurring | Fair Value, Inputs, Level 3
Assets:
Other assets	50,776		51,428
Other assets - SBIC investments | Fair Value, measurements, recurring
Assets:
Other assets	22,553		15,639
Other assets - SBIC investments | Fair Value, measurements, recurring | Fair Value, Inputs, Level 3
Assets:
Other assets	$ 22,553		$ 15,639

[1] Excludes $400 million of FHLB and Federal Reserve stock required to be owned by the Company at March 31, 2017. These securities are carried at par.

[2] Excludes $403 million of FHLB and Federal Reserve stock required to be owned by the Company at December 31, 2016. These securities are carried at par.