Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fees and Commissions, Mortgage Banking and Servicing	$ 30,400	$ 24,900	$ 28,900
Noninterest Income, Other Operating Income	15,900	15,100	14,300
Noninterest Income	149,900	143,300	141,800
Loss on termination of interest rate swap	0	0	(1,100)
Cash flows from operating activities:
Net income	181,000	160,200	106,500
Adjustments to reconcile net income from operations to net cash provided by operating activities:
Provision for loan losses	13,900	8,600	11,000
Net (gain) loss on disposal of property and equipment	(1,500)	(1,200)	200
Depreciation and amortization	38,700	27,600	18,400
Net premium amortization on investment securities	8,900	10,000	11,600
Net (gain) loss on investment securities transactions	(100)	100	(700)
Realized and unrealized net gains on mortgage banking activities	(30,500)	(23,000)	(25,200)
Net loss (gain) on sale of OREO	(3,600)	(800)	100
Write-downs of OREO and other assets pending disposal	900	100	400
Net (gain) on sale of health savings accounts	0	0	(3,100)
Mortgage servicing rights impairment (recovery)	400	0	(100)
Deferred income tax expense	5,400	15,800	20,900
Net increase in cash surrender value of company-owned life insurance policies	(6,700)	(5,000)	(5,400)
Stock-based compensation expense	8,000	5,600	3,900
Originations of mortgage loans held for sale	(1,015,600)	(768,100)	(815,000)
Proceeds from sales of mortgage loans held for sale	971,200	798,400	860,100
Changes in operating assets and liabilities:
Decrease (increase) in interest receivable	300	(3,300)	(600)
Increase in other assets	(22,100)	(8,000)	(6,100)
(Decrease) increase in interest payable	(13,500)	2,200	200
Decrease in accounts payable and accrued expenses	(7,800)	(200)	(22,500)
Net cash provided by operating activities	127,300	219,000	154,600
Purchases of investment securities:
Held-to-maturity	0	(2,000)	(12,800)
Payments to Acquire Debt Securities, Available-for-sale	1,270,000	541,000
Available-for-sale			(614,300)
Proceeds from maturities, pay-downs, calls and sales of investment securities:
Held-to-maturity	35,600	79,300	97,400
Available-for-sale	978,600	460,000	426,200
Proceeds from bank-owned life insurance settlements	3,200	0	0
Extensions of credit to clients, net of repayments	(81,400)	(221,700)	(99,700)
Payments For (Proceeds From) Recoveries of Loans Previously Charged off	(9,700)	12,000
Recoveries of loans charged-off			7,800
Proceeds from sales of OREO	25,400	9,100	5,900
Acquisition of intangible assets	0	0	(28,000)
Proceeds from the sale of health savings accounts	300	0	6,100
Acquisition of banks and bank holding companies, net of cash and cash equivalents acquired	(298,400)	28,100	(91,800)
Capital expenditures, net of proceeds from sales	(16,600)	(4,900)	(11,000)
Net cash used in investing activities	16,800	(181,100)	130,600
Cash flows from financing activities:
Net increase (decrease) in deposits	276,200	49,500	(110,200)
Net increase (decrease) in securities sold under repurchase agreements	(45,200)	69,400	105,400
Net increase (decrease) in other borrowed funds	(4,100)	(26,100)	100
Repayments of long-term debt	(2,000)	(7,100)	100
Advances on long-term debt	100	2,800	5,000
Proceeds from issuance of common stock	1,000	1,800	2,400
Purchase and retirement of common stock	(2,500)	(1,000)	(1,300)
Dividends paid to common stockholders	(79,200)	(64,100)	(48,600)
Net cash used in financing activities	144,300	25,200	(47,100)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	254,800	63,100
Net change in cash and cash equivalents			(23,100)
Cash and cash equivalents at beginning of period		758,900	782,000
Cash and cash equivalents at end of period			758,900
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	(51,200)	(25,300)	(28,800)
Interest Paid, Excluding Capitalized Interest, Operating Activities	54,700	38,700
Cash paid during the period for interest expense			(27,800)
Transfer of Portfolio Loans and Leases to Held-for-sale	0	1,600	3,000
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Before Tax	281,100
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	(6,000)	0	0
Right-of-Use Asset Obtained in Exchange for Finance Lease Liability	39,600	0	0
Supplemental disclosures of noncash investing and financing activities:
Transfer from long-term debt to other borrowed funds	0	0	20,000
Transfer of loans to other real estate owned	14,100	12,100	5,400
Capitalization of internally originated mortgage servicing rights	7,300	6,100	5,600
Supplemental schedule of noncash investing activities from acquisitions:
Investment securities available for sale	78,700	3,100	424,300
Investment securities held to maturity	0	0	57,300
Loans held for sale	500	0	10,300
Loans	416,600	713,100	2,079,300
Premises and equipment	24,600	14,000	47,700
Goodwill	75,300	101,100	231,900
Core deposit intangible	16,600	15,700	48,000
Mortgage servicing rights	0	0	3,500
Deferred tax assets	2,200	3,600	7,600
Interest receivable	15,200	9,500	57,000
Deferred tax assets	0	0	28,600
Other real estate owned	2,400	600	1,200
Other assets	6,500	6,200	31,600
Total noncash assets acquired	638,600	866,900	3,028,300
Deposits	706,700	696,300	2,669,000
Noncash Or Part Noncash Acquisition, Securities Sold Under Repurchase Agreements	30,400	0	0
Deferred tax liability	19,900	7,700	64,200
Noncash or Part Noncash Acquisition, Debt Assumed	4,100	7,000	0
Noncash or Part Noncash Acquisition, Other Liabilities Assumed	0	6,100	0
Noncash or Part Noncash Acquisition, Trust Preferred Securities Assumed	0	4,400	0
Noncash or Part Noncash Acquisition, Deferred Tax Liability	100	300	0
Total liabilities assumed	(761,200)	(721,800)	(2,733,200)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	1,076,800	822,000	758,900
Credit and Debit Card [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	41,500	43,300	43,300
Investment Advisory, Management and Administrative Service [Member]
Investment Banking, Advisory, Brokerage, and Underwriting Fees and Commissions	23,800	23,200	21,100
Deposit Account [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	21,100	21,800	21,300
Financial Service, Other [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 17,100	$ 15,100	$ 13,300