Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Assets [Abstract]
Cash and due from banks	$ 561,587	$ 620,846	$ 498,713
Interest-bearing cash and cash equivalents	2,078,831	1,916,651	1,907,838
Trading securities	441,414	337,628	211,622
Investment securities	490,426	546,145	560,711
Available for sale securities	8,341,041	8,676,829	8,830,689
Fair value option securities	445,169	77,046	263,265
Restricted equity securities	311,033	307,240	319,639
Residential mortgage loans held for sale	287,259	301,897	430,728
Loans	17,183,645	16,989,660	16,406,749
Allowance for loan losses	(250,061)	(246,159)	(243,259)
Loans, net of allowance	16,933,584	16,743,501	16,163,490
Premises and equipment, net	321,038	325,849	315,199
Receivables	295,042	772,952	173,638
Goodwill	446,697	448,899	382,739
Intangible assets, net	40,755	46,931	43,372
Mortgage servicing rights	245,239	247,073	190,747
Real estate and other repossessed assets, net of allowance	39,436	44,287	24,054
Derivative contracts, net	280,289	689,872	883,673
Cash surrender value of bank-owned life insurance	312,774	308,430	307,860
Receivable on unsettled securities sales	33,177	7,188	142,820
Other assets	358,741	353,017	319,653
Total assets	32,263,532	32,772,281	31,970,450
Deposits [Abstract]
Noninterest-bearing demand deposits	9,568,895	9,235,720	8,424,609
Interest-bearing deposits: [Abstract]
Transaction	10,087,139	10,865,105	9,668,869
Savings	464,318	425,470	419,262
Time	2,196,122	2,221,800	2,247,061
Total deposits	22,316,474	22,748,095	20,759,801
Funds purchased	67,990	57,929	56,780
Repurchase agreements	396,333	668,661	472,683
Other borrowings	5,232,343	4,846,072	5,830,736
Subordinated debentures	144,658	144,640	371,812
Accrued interest, taxes and expense	133,198	146,704	197,742
Derivative contracts, net	285,819	664,531	719,159
Due on unsettled securities purchases	32,636	6,508	11,757
Other liabilities	204,536	182,784	147,242
Total liabilities	28,813,987	29,465,924	28,567,712
Shareholders' equity: [Abstract]
Common stock	4	4	4
Capital surplus	1,017,495	1,006,535	990,106
Retained earnings	2,942,447	2,823,334	2,755,766
Treasury stock	(545,441)	(544,052)	(494,675)
Accumulated other comprehensive income (loss)	7,964	(10,967)	117,632
Total shareholders’ equity	3,422,469	3,274,854	3,368,833
Non-controlling interests	27,076	31,503	33,905
Total equity	3,449,545	3,306,357	3,402,738
Total liabilities and equity	$ 32,263,532	$ 32,772,281	$ 31,970,450