Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Assets [Abstract]
Cash and due from banks	$ 547,203	$ 620,846	$ 535,916
Interest-bearing cash and cash equivalents	1,926,779	1,916,651	2,080,978
Trading securities	614,117	337,628	546,615
Investment securities	466,562	546,145	546,457
Available for sale securities	8,383,199	8,676,829	8,862,283
Fair value option securities	819,531	77,046	222,409
Restricted equity securities	347,542	307,240	333,391
Residential mortgage loans held for sale	275,643	301,897	447,592
Loans	17,206,834	16,989,660	16,464,786
Allowance for loan losses	(247,703)	(246,159)	(245,103)
Loans, net of allowance	16,959,131	16,743,501	16,219,683
Premises and equipment, net	320,060	325,849	318,196
Receivables	314,251	772,952	650,368
Goodwill	446,697	448,899	382,739
Intangible assets, net	39,013	46,931	41,977
Mortgage servicing rights	245,858	247,073	203,621
Real estate and other repossessed assets, net of allowance	32,535	44,287	31,941
Derivative contracts, net	352,559	689,872	655,078
Cash surrender value of bank-owned life insurance	314,201	308,430	310,211
Receivable on unsettled securities sales	230,225	7,188	19,642
Other assets	370,409	353,017	370,134
Total assets	33,005,515	32,772,281	32,779,231
Deposits [Abstract]
Noninterest-bearing demand deposits	9,185,481	9,235,720	8,681,364
Interest-bearing deposits: [Abstract]
Transaction	10,025,084	10,865,105	9,824,160
Savings	465,225	425,470	420,349
Time	2,172,289	2,221,800	2,169,631
Total deposits	21,848,079	22,748,095	21,095,504
Funds purchased	62,356	57,929	109,031
Repurchase agreements	328,189	668,661	504,573
Other borrowings	6,241,275	4,846,072	6,533,443
Subordinated debentures	144,668	144,640	144,631
Accrued interest, taxes and expense	152,029	146,704	191,276
Derivative contracts, net	336,327	664,531	573,987
Due on unsettled securities purchases	160,781	6,508	677
Other liabilities	217,372	182,784	193,698
Total liabilities	29,491,076	29,465,924	29,346,820
Shareholders' equity: [Abstract]
Common stock	4	4	4
Capital surplus	1,028,489	1,006,535	995,680
Retained earnings	2,999,005	2,823,334	2,801,931
Treasury stock	(545,441)	(544,052)	(495,031)
Accumulated other comprehensive income (loss)	6,757	(10,967)	95,727
Total shareholders’ equity	3,488,814	3,274,854	3,398,311
Non-controlling interests	25,625	31,503	34,100
Total equity	3,514,439	3,306,357	3,432,411
Total liabilities and equity	$ 33,005,515	$ 32,772,281	$ 32,779,231