Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Assets [Abstract]
Cash and due from banks	$ 585,801	$ 602,510	$ 561,587
Interest-bearing cash and cash equivalents	872,999	1,714,544	2,078,831
Trading securities	1,909,615	462,676	441,414
Investment securities	392,013	461,793	490,426
Available for sale securities	8,162,866	8,321,578	8,341,041
Fair value option securities	482,227	755,054	445,169
Restricted equity securities	347,721	320,189	311,033
Residential mortgage loans held for sale	223,301	221,378	287,259
Loans	18,003,696	17,153,424	17,183,645
Allowance for loan losses	(215,142)	(230,682)	(250,061)
Loans, net of allowance	17,788,554	16,922,742	16,933,584
Premises and equipment, net	320,810	317,335	321,038
Receivables	212,893	178,800	170,094
Goodwill	453,093	447,430	446,697
Intangible assets, net	28,273	28,658	40,755
Mortgage servicing rights	278,719	252,867	245,239
Real estate and other repossessed assets, net of allowance	27,891	28,437	39,436
Derivative contracts, net	373,373	220,502	280,289
Cash surrender value of bank-owned life insurance	321,024	316,498	312,774
Receivable on unsettled securities sales	604,552	340,077	158,125
Other assets	447,382	359,092	358,741
Total assets	33,833,107	32,272,160	32,263,532
Deposits [Abstract]
Noninterest-bearing demand deposits	9,373,959	9,243,338	9,568,895
Interest-bearing deposits: [Abstract]
Transaction	10,164,099	10,250,393	10,087,139
Savings	503,474	469,158	464,318
Time	2,127,732	2,098,416	2,196,122
Total deposits	22,169,264	22,061,305	22,316,474
Funds purchased and repurchase agreements	880,027	574,964	464,323
Other borrowings	5,929,445	5,134,897	5,232,343
Subordinated debentures	144,697	144,677	144,658
Accrued interest, taxes and expense	160,568	164,895	133,198
Derivative contracts, net	234,856	171,963	285,819
Due on unsettled securities purchases	571,034	338,745	31,214
Other liabilities	167,171	162,380	205,958
Total liabilities	30,257,062	28,753,826	28,813,987
Shareholders' equity: [Abstract]
Common stock	4	4	4
Capital surplus	1,040,202	1,035,895	1,017,495
Retained earnings	3,212,653	3,048,487	2,942,447
Treasury stock	(564,123)	(552,845)	(545,441)
Accumulated other comprehensive gain (loss)	(135,305)	(36,174)	7,964
Total shareholders’ equity	3,553,431	3,495,367	3,422,469
Non-controlling interests	22,614	22,967	27,076
Total equity	3,576,045	3,518,334	3,449,545
Total liabilities and equity	$ 33,833,107	$ 32,272,160	$ 32,263,532