Fair Value Measurements, Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2015		Jun. 30, 2015	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest-bearing cash and cash equivalents	$ 1,830,105			$ 1,925,266		$ 2,007,901
Trading securities	181,131			188,700		169,712
Investment securities	612,384	[1]		652,360	[2]	655,091	[3]
Available for sale securities	8,801,089			8,978,945		9,306,886
Fair value option securities	427,760			311,597		175,761
Residential mortgage loans held for sale	357,414			304,182		373,253
Loans	15,367,441			14,208,037		13,683,739
Allowance for loan losses	(204,116)		$ (201,087)	(189,056)		(191,244)		$ (190,690)	$ (185,396)
Loans, net of allowance	15,163,325			14,018,981		13,492,495
Mortgage servicing rights	200,049		198,694	171,976		173,286		155,740	153,333
Derviative contracts, net of cash margin, Assets, Fair value	726,159			361,874		360,809
Time deposits	2,498,531			2,608,716		2,664,580
Subordinated debentures	226,314			347,983		347,936
Derivative contracts, net of cash margin, Liabilities, Fair Value	636,115			354,554		348,687
Fair Value Assumptions and Methodology for Assets and Liabilities [Abstract]
Specific allocation of allowance for loan losses included in fair value of loans	176,000			161,000		163,000
Commercial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	9,797,422			9,095,670		8,572,038
Allowance for loan losses	(108,993)		(107,037)	(90,875)		(86,775)		(87,806)	(79,180)
Commercial Real Estate [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,235,067			2,728,150		2,724,199
Allowance for loan losses	(41,789)		(39,744)	(42,445)		(42,433)		(41,252)	(41,573)
Residential Mortgage [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	1,868,995			1,949,512		1,979,663
Allowance for loan losses	(20,859)		(21,449)	(23,458)		(27,216)		(27,654)	(29,465)
Personal [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	465,957			434,705		407,839
Allowance for loan losses	(4,729)		$ (3,955)	(4,233)		(6,876)		$ (7,029)	$ (6,965)
U.S. Government agency debentures [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	42,431			85,092		41,004
U.S. agency residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	30,973			31,199		33,226
Investment securities	28,653
Available for sale securities	5,819,127			6,646,884		6,850,603
Fair value option securities	427,760			311,597		175,761
Municipal and other tax-exempt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	84,261			38,951		76,884
Investment securities	379,980	[1]		405,090	[2]	410,595	[3]
Available for sale securities	57,960			63,557		64,363
Other trading securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	23,466			33,458		18,598
U.S. Treasury [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	1,003			1,005		1,015
Privately issued residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	145,682			165,957		171,493
Commercial mortgage-backed securities guaranteed by U.S. government agencies [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	2,735,787			2,048,609		2,141,645
Other debt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities	203,751	[1]		211,520	[2]	205,911	[3]
Available for sale securities	4,150			9,212		34,291
Perpetual preferred stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	19,163			24,277		24,358
Equity securities and mutual funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	18,217			19,444		19,118
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks	489,268			550,576		557,658
Interest-bearing cash and cash equivalents	1,830,105			1,925,266		2,007,901
Trading securities	181,131			188,700		169,712
Investment securities	612,384			652,360		655,091
Available for sale securities	8,801,089			8,978,945		9,306,886
Fair value option securities	427,760			311,597		175,761
Residential mortgage loans held for sale	357,414			304,182		373,253
Loans	15,367,441			14,208,037		13,683,739
Allowance for loan losses	(204,116)			(189,056)		(191,244)
Loans, net of allowance	15,163,325			14,018,981		13,492,495
Mortgage servicing rights	200,049			171,976		173,286
Derviative contracts, net of cash margin, Assets, Fair value	726,159			361,874		360,809
Other assets, private equity funds	24,133			25,627		27,118
Deposits with no stated maturity	18,120,912			18,532,143		17,624,476
Time deposits	2,498,531			2,608,716		2,664,580
Other borrowed funds	5,253,124			3,378,294		4,595,631
Subordinated debentures	226,314			347,983		347,936
Derivative contracts, net of cash margin, Liabilities, Fair Value	636,115			354,554		348,687
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Commercial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	9,797,422			9,095,670		8,572,038
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Commercial Real Estate [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,235,067			2,728,150		2,724,199
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Residential Mortgage [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	1,868,995			1,949,512		1,979,663
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Personal [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	465,957			434,705		407,839
Carrying (Reported) Amount, Fair Value Disclosure [Member] | U.S. Government agency debentures [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	42,431			85,092		41,004
Carrying (Reported) Amount, Fair Value Disclosure [Member] | U.S. agency residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	30,973			31,199		33,226
Investment securities	28,653			35,750		38,585
Available for sale securities	5,819,127			6,646,884		6,850,603
Fair value option securities	427,760			311,597		175,761
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Municipal and other tax-exempt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	84,261			38,951		76,884
Investment securities	379,980			405,090		410,595
Available for sale securities	57,960			63,557		64,363
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Other trading securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	23,466			33,458		18,598
Carrying (Reported) Amount, Fair Value Disclosure [Member] | U.S. Treasury [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	1,003			1,005		1,015
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Privately issued residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	145,682			165,957		171,493
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Commercial mortgage-backed securities guaranteed by U.S. government agencies [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	2,735,787			2,048,609		2,141,645
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Other debt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities	203,751			211,520		205,911
Available for sale securities	4,150			9,212		34,291
Fair value option securities	0			0		0
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Perpetual preferred stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	19,163			24,277		24,358
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Equity securities and mutual funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	18,217			19,444		19,118
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks	489,268			550,576		557,658
Interest-bearing cash and cash equivalents	1,830,105			1,925,266		2,007,901
Trading securities	181,131			188,700		169,712
Investment securities	643,091			673,626		676,445
Available for sale securities	8,801,089			8,978,945		9,306,886
Fair value option securities	427,760			311,597		175,761
Residential mortgage loans held for sale	357,414			304,182		373,253
Loans	15,229,586			14,070,468		13,555,114
Allowance for loan losses	0			0		0
Loans, net of allowance	15,229,586			14,070,468		13,555,114
Mortgage servicing rights	200,049			171,976		173,286
Derviative contracts, net of cash margin, Assets, Fair value	726,159			361,874		360,809
Other assets, private equity funds	24,133			25,627		27,118
Deposits with no stated maturity	18,120,912			18,532,143		17,624,476
Time deposits	2,500,469			2,612,576		2,670,657
Other borrowed funds	5,239,400			3,331,771		4,555,307
Subordinated debentures	223,334			344,687		344,764
Derivative contracts, net of cash margin, Liabilities, Fair Value	636,115			354,554		348,687
Estimate of Fair Value, Fair Value Disclosure [Member] | Commercial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	9,530,437			8,948,870		8,441,120
Estimate of Fair Value, Fair Value Disclosure [Member] | Commercial Real Estate [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,330,298			2,704,454		2,702,389
Estimate of Fair Value, Fair Value Disclosure [Member] | Residential Mortgage [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	1,906,585			1,985,870		2,009,619
Estimate of Fair Value, Fair Value Disclosure [Member] | Personal [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	462,266			431,274		401,986
Estimate of Fair Value, Fair Value Disclosure [Member] | U.S. Government agency debentures [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	42,431			85,092		41,004
Estimate of Fair Value, Fair Value Disclosure [Member] | U.S. agency residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	30,973			31,199		33,226
Investment securities	30,080			37,463		40,259
Available for sale securities	5,819,127			6,646,884		6,850,603
Fair value option securities	427,760			311,597		175,761
Estimate of Fair Value, Fair Value Disclosure [Member] | Municipal and other tax-exempt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	84,261			38,951		76,884
Investment securities	384,310			408,344		415,233
Available for sale securities	57,960			63,557		64,363
Estimate of Fair Value, Fair Value Disclosure [Member] | Other trading securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	23,466			33,458		18,598
Estimate of Fair Value, Fair Value Disclosure [Member] | U.S. Treasury [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	1,003			1,005		1,015
Estimate of Fair Value, Fair Value Disclosure [Member] | Privately issued residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	145,682			165,957		171,493
Estimate of Fair Value, Fair Value Disclosure [Member] | Commercial mortgage-backed securities guaranteed by U.S. government agencies [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	2,735,787			2,048,609		2,141,645
Estimate of Fair Value, Fair Value Disclosure [Member] | Other debt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities	228,701			227,819		220,953
Available for sale securities	4,150			9,212		34,291
Fair value option securities	0			0		0
Estimate of Fair Value, Fair Value Disclosure [Member] | Perpetual preferred stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	19,163			24,277		24,358
Estimate of Fair Value, Fair Value Disclosure [Member] | Equity securities and mutual funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	$ 18,217			$ 19,444		$ 19,118

[1] Carrying value includes $197 thousand of net unrealized gain which remains in Accumulated other comprehensive income (“AOCI”) in the Consolidated Balance Sheets related to certain securities transferred from the Available for Sale securities portfolio to the Investment securities portfolio in 2011.

[2] Carrying value includes $615 thousand of net unrealized gain which remains in Accumulated other comprehensive income (“AOCI”) in the Consolidated Balance Sheets related to certain securities transferred from the Available for Sale securities portfolio to the Investment securities portfolio in 2011.

[3] Carrying value includes $822 thousand of net unrealized gain which remains in Accumulated other comprehensive income (“AOCI”) in the Consolidated Balance Sheets related to certain securities transferred from the Available for Sale securities portfolio to the Investment securities portfolio in 2011.