Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Assets [Abstract]
Cash and due from banks	$ 498,713	$ 573,699	$ 443,577
Interest-bearing cash and cash equivalents	1,907,838	2,069,900	2,119,072
Trading securities	211,622	122,404	158,209
Investment securities	560,711	597,836	625,664
Available for sale securities	8,830,689	9,042,733	9,000,117
Fair value option securities	263,265	444,217	436,324
Restricted equity securities	319,639	273,684	231,520
Residential mortgage loans held for sale	430,728	308,439	502,571
Loans	16,406,749	15,941,154	15,124,136
Allowance for loan losses	(243,259)	(225,524)	(201,087)
Loans, net of allowance	16,163,490	15,715,630	14,923,049
Premises and equipment, net	315,199	306,490	284,238
Receivables	173,638	163,480	149,629
Goodwill	382,739	385,461	385,454
Intangible assets, net	43,372	43,909	46,061
Mortgage servicing rights	190,747	218,605	198,694
Real estate and other repossessed assets, net of allowance	24,054	30,731	35,499
Derivative contracts, net	883,673	586,270	630,435
Cash surrender value of bank-owned life insurance	307,860	303,335	298,606
Receivable on unsettled securities sales	142,820	40,193	8,693
Other assets	319,653	249,112	248,151
Total assets	31,970,450	31,476,128	30,725,563
Deposits [Abstract]
Noninterest-bearing demand deposits	8,424,609	8,296,888	8,156,401
Interest-bearing deposits: [Abstract]
Transaction	9,668,869	9,998,954	9,899,777
Savings	419,262	386,252	379,172
Time	2,247,061	2,406,064	2,624,379
Total deposits	20,759,801	21,088,158	21,059,729
Funds purchased	56,780	491,192	64,677
Repurchase agreements	472,683	722,444	712,033
Other borrowings	5,830,736	4,837,879	4,332,162
Subordinated debentures	371,812	226,350	226,278
Accrued interest, taxes and expense	197,742	119,584	124,568
Derivative contracts, net	719,159	581,701	620,277
Due on unsettled securities purchases	11,757	16,897	37,571
Other liabilities	147,242	124,284	135,435
Total liabilities	28,567,712	28,208,489	27,312,730
Shareholders' equity: [Abstract]
Common stock	4	4	4
Capital surplus	990,106	982,009	970,054
Retained earnings	2,755,766	2,704,121	2,627,250
Treasury stock	(494,675)	(477,165)	(273,468)
Accumulated other comprehensive income	117,632	21,587	51,792
Total shareholders' equity	3,368,833	3,230,556	3,375,632
Non-controlling interests	33,905	37,083	37,201
Total equity	3,402,738	3,267,639	3,412,833
Total liabilities and equity	$ 31,970,450	$ 31,476,128	$ 30,725,563