Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Assets [Abstract]
Cash and due from banks	$ 544,534	$ 602,510	$ 546,575
Interest-bearing cash and cash equivalents	2,054,899	1,714,544	2,220,640
Trading securities	1,292,432	462,676	677,156
Investment securities	416,672	461,793	519,402
Available for sale debt securities	8,249,432	8,321,578	8,437,291
Fair value option securities	513,668	755,054	441,714
Restricted equity securities	338,552	320,189	283,936
Residential mortgage loans held for sale	225,190	221,378	248,707
Loans	17,337,850	17,153,424	16,991,906
Allowance for loan losses	(223,967)	(230,682)	(248,710)
Loans, net of allowance	17,113,883	16,922,742	16,743,196
Premises and equipment, net	314,347	317,335	325,546
Receivables	478,027	442,897	394,394
Goodwill	447,430	447,430	445,738
Intangible assets, net	29,658	28,658	42,556
Mortgage servicing rights	274,978	252,867	249,403
Real estate and other repossessed assets, net of allowance	23,652	28,437	42,726
Derivative contracts, net	286,687	220,502	304,727
Cash surrender value of bank-owned life insurance	318,661	316,498	310,537
Receivable on unsettled securities sales	3,638	75,980	9,921
Other assets	435,152	359,092	384,767
Total assets	33,361,492	32,272,160	32,628,932
Deposits [Abstract]
Noninterest-bearing demand deposits	9,306,023	9,243,338	9,506,573
Interest-bearing deposits: [Abstract]
Transaction	10,226,971	10,250,393	10,359,214
Savings	505,952	469,158	465,724
Time	2,166,254	2,098,416	2,243,848
Total deposits	22,205,200	22,061,305	22,575,359
Funds purchased	130,561	58,628	47,629
Repurchase agreements	415,763	516,335	508,352
Other borrowings	5,727,025	5,134,897	5,238,947
Subordinated debentures	144,687	144,677	144,649
Accrued interest, taxes and expense	156,146	164,895	140,235
Derivative contracts, net	233,202	171,963	276,422
Due on unsettled securities purchases	94,424	151,198	137,069
Other liabilities	737,142	349,928	189,376
Total liabilities	29,844,150	28,753,826	29,258,038
Shareholders' equity: [Abstract]
Common stock	4	4	4
Capital surplus	1,041,242	1,035,895	1,009,360
Retained earnings	3,127,575	3,048,487	2,883,042
Treasury stock	(562,601)	(552,845)	(545,441)
Accumulated other comprehensive loss	(111,191)	(36,174)	(5,221)
Total shareholders’ equity	3,495,029	3,495,367	3,341,744
Non-controlling interests	22,313	22,967	29,150
Total equity	3,517,342	3,518,334	3,370,894
Total liabilities and equity	$ 33,361,492	$ 32,272,160	$ 32,628,932