Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OPERATING ACTIVITIES
Net income	$ 247,105	$ 158,801	$ 116,073
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for loan losses	41,000	32,500	15,500
Net accretion of premiums and discounts from acquisition	(1,906)	(2,303)	(2,727)
Depreciation and amortization	54,875	54,556	52,751
Deferred income tax expense (benefit)	59,738	2,756	(4,848)
Net (increase) decrease in trading securities and other earning assets	(10,805)	(12,420)	10,258
Gains on sales of securities available for sale, net	(4,192)	(8,509)	(10,402)
Gains on sales of assets	(103,346)	(762)	(98)
Amortization of securities premiums, net of discount accretion	48,101	54,467	57,301
Originations of loans held for sale	(65,163)	(92,438)	(96,324)
Gains on sales of loans held for sale, net	(1,561)	(1,774)	(1,331)
Proceeds from sales of loans held for sale	70,543	89,522	97,690
Equity based compensation	13,316	11,735	10,751
Net tax benefit related to equity compensation plans	3,612	1,073	944
Changes in:
Accrued income	(9,201)	(8,918)	(7,075)
Accrued expenses and taxes	(40,806)	14,112	(4,503)
Other assets and liabilities, net	25,216	4,042	(22,055)
Net cash provided by operating activities	326,526	296,440	211,905
INVESTING ACTIVITIES
Proceeds from maturities of securities held to maturity	87,595	48,539	59,775
Proceeds from sales of securities available for sale	578,517	951,264	946,045
Proceeds from maturities of securities available for sale	1,198,834	1,792,357	1,200,178
Purchases of securities held to maturity	(236,832)	(500,682)	(451,350)
Purchases of securities available for sale	(1,585,395)	(2,546,028)	(1,923,747)
Net increase in loans	(770,727)	(1,129,026)	(988,434)
Net decrease (increase) in fed funds sold and resell agreements	132,726	(150,700)	(45,190)
Net cash activity from acquisitions and divestitures	164,561		95,351
Net decrease in interest bearing balances due from other financial institutions	45,752	88,009	34,473
Purchases of premises and equipment	(36,447)	(50,841)	(53,760)
Proceeds from sales of premises and equipment	3,037	1,760	1,069
Purchases of bank-owned and company-owned life insurance	(62,800)	(7,095)	(204,647)
Proceeds from bank-owned life insurance death benefit	2,601
Net cash used in investing activities	(478,578)	(1,502,443)	(1,330,237)
FINANCING ACTIVITIES
Net increase in demand and savings deposits	1,307,843	1,598,026	894,667
Net increase (decrease) in time deposits	144,543	(119,315)	(352,622)
Net (decrease) increase in fed funds purchased and repurchase agreements	(596,233)	38,875	(207,070)
Net decrease in short-term debt		(5,000)	(112,133)
Proceeds from long-term debt	3,003	1,500	2,500
Repayment of long-term debt	(1,524)	(11,703)	(10,816)
Payment of contingent consideration on acquisitions		(3,031)	(21,494)
Cash dividends paid	(51,876)	(49,038)	(45,967)
Proceeds from exercise of stock options and sales of treasury shares	13,867	16,911	11,606
Purchases of treasury stock	(15,276)	(16,367)	(8,457)
Net cash (used in) provided by financing activities	804,347	1,450,858	150,214
Increase (decrease) in cash and cash equivalents	652,295	244,855	(968,118)
Cash and cash equivalents at beginning of year	1,063,967	819,112	1,787,230
Cash and cash equivalents at end of year	1,716,262	1,063,967	819,112
Supplemental disclosures:
Income taxes paid	45,749	44,076	47,086
Total interest paid	$ 56,820	$ 27,999	17,812
Transactions related to Marquette acquisition
Assets acquired			1,312,174
Liabilities assumed			$ 1,151,025