Consolidated Balance Sheets (Unaudited) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014
Assets [Abstract]
Cash and due from banks	$ 490,683		$ 550,576		$ 645,435
Interest-bearing cash and cash equivalents	2,119,987		1,925,266		708,571
Trading securities	118,044		188,700		86,571
Investment securities	634,587	[1]	652,360	[2]	668,976	[3]
Available for sale securities	9,158,175		8,978,945		9,933,723
Fair value option securities	434,077		311,597		160,884
Restricted equity securities	212,685		141,494		85,643
Residential mortgage loans held for sale	513,196		304,182		226,512
Loans	14,684,136		14,208,037		13,077,854
Allowance for loan losses	(197,686)		(189,056)		(188,318)
Loans, net of allowance	14,486,450		14,018,981		12,889,536
Premises and equipment, net	279,075		273,833		279,257
Receivables	183,447		132,408		114,437
Goodwill	377,780		377,780		364,570
Intangible assets, net	33,286		34,376		31,561
Mortgage servicing rights	175,051		171,976		153,774
Real estate and other repossessed assets, net of allowance	45,551		101,861		95,515
Derivative contracts	462,386		361,874		218,507
Cash surrender value of bank-owned life insurance	296,192		293,978		286,932
Receivable on unsettled securities sales	9,598		74,259		18,199
Other assets	269,728		195,252		396,111
Total assets	30,299,978		29,089,698		27,364,714
Deposits [Abstract]
Noninterest-bearing demand deposits	8,009,577		8,066,357		7,472,287
Interest-bearing deposits: [Abstract]
Transaction	10,108,202		10,114,355		9,899,656
Savings	383,790		351,431		355,596
Time	2,651,778		2,608,716		2,662,174
Total deposits	21,153,347		21,140,859		20,389,713
Funds purchased	66,320		57,031		1,166,178
Repurchase agreements	897,663		1,187,489		777,108
Other borrowings	3,727,050		2,133,774		1,031,693
Subordinated debentures	348,030		347,983		347,846
Accrued interest, taxes and expense	147,184		120,211		160,351
Derivative contracts	419,351		354,554		185,499
Due on unsettled securities purchases	25,935		290,540		39,641
Other liabilities	124,846		121,051		122,086
Total liabilities	26,909,726		25,753,492		24,220,115
Shareholders' equity: [Abstract]
Common stock	4		4		4
Capital surplus	959,650		954,644		913,642
Retained earnings	2,576,953		2,530,837		2,398,636
Treasury stock	(269,749)		(239,979)		(209,152)
Accumulated other comprehensive income	90,303		56,673		6,795
Total shareholders' equity	3,357,161		3,302,179		3,109,925
Non-controlling interests	33,091		34,027		34,674
Total equity	3,390,252		3,336,206		3,144,599
Total liabilities and equity	$ 30,299,978		$ 29,089,698		$ 27,364,714

[1] Carrying value includes $436 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 $1.4 million 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.