Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income	$ 161,200	$ 181,000	$ 160,200
Adjustments to reconcile net income from operations to net cash provided by operating activities:
Provision for credit losses	56,900	13,900	8,600
Net loss (gain) on disposal of property and equipment	300	(1,500)	(1,200)
Depreciation and amortization	45,100	38,700	27,600
Net premium amortization on investment securities	15,900	8,900	10,000
Net (gain) loss on investment securities transactions	(300)	(100)	100
Realized and unrealized net gains on mortgage banking activities	(49,300)	(30,500)	(23,000)
Net gain on sale of investments in unrelated entities	(1,000)	0	0
Net gain on sale of OREO	(900)	(3,600)	(800)
Write-downs of OREO and other assets pending disposal	100	900	100
Mortgage servicing rights impairment (recovery)	9,900	400	0
Deferred taxes	(6,600)	5,400	15,800
Net increase in cash surrender value of company-owned life insurance policies	(7,600)	(6,700)	(5,000)
Stock-based compensation expense	7,500	8,000	5,600
Originations of mortgage loans held for sale	(1,404,200)	(1,015,600)	(768,100)
Proceeds from sales of mortgage loans held for sale	1,468,400	971,200	798,400
Changes in operating assets and liabilities:
(Increase) decrease in interest receivable	(4,400)	300	(3,300)
Increase in other assets	(27,700)	(22,100)	(8,000)
(Decrease) increase in interest payable	(6,300)	(13,500)	2,200
Increase (decrease) in accounts payable and accrued expenses	11,300	(7,800)	(200)
Net cash provided by operating activities	268,300	127,300	219,000
Purchases of investment securities:
Held-to-maturity	0	0	(2,000)
Available-for-sale	(2,444,100)	(1,270,000)	(541,000)
Proceeds from maturities, pay-downs, calls and sales of investment securities:
Held-to-maturity	40,400	35,600	79,300
Available-for-sale	1,441,400	978,600	460,000
Proceeds from bank-owned life insurance settlements	5,000	3,200	0
Extensions of credit to clients, net of repayments	(901,300)	(81,400)	(221,700)
Recoveries of loans charged-off	6,700	9,700	12,000
Proceeds from sales of OREO	10,100	25,400	9,100
Proceeds from the sale of health savings accounts	0	300	0
Proceeds from sale of investments in unrelated entities	2,200	0	0
Acquisition of banks and bank holding companies, net of cash and cash equivalents received	0	298,400	28,100
Capital expenditures, net of proceeds from sales	(30,200)	(16,600)	(4,900)
Net cash used in investing activities	(1,869,800)	(16,800)	(181,100)
Cash flows from financing activities:
Net increase in deposits	2,553,500	276,200	49,500
Net increase (decrease) in securities sold under repurchase agreements	393,800	(45,200)	69,400
Net decrease in other borrowed funds	0	(4,100)	(26,100)
Repayments of long-term debt	(100)	(2,000)	(7,100)
Advances on long-term debt	98,600	100	2,800
Proceeds from issuance of common stock	1,100	1,000	1,800
Purchase and retirement of common stock	(116,800)	(2,500)	(1,000)
Dividends paid to common stockholders	(128,600)	(79,200)	(64,100)
Net cash used in financing activities	2,801,500	144,300	25,200
Net increase (decrease) in cash and cash equivalents	1,200,000	254,800	63,100
Cash and cash equivalents at beginning of period	1,076,800	822,000	758,900
Cash and cash equivalents at end of period	2,276,800	1,076,800	822,000
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	(54,400)	(51,200)	(25,300)
Cash paid during the period for interest expense	32,900	54,700	38,700
Supplemental disclosures of noncash investing and financing activities:
Amortization of unrealized gains and losses on transfers of securities	0	0	1,600
Transfer of securities from held-to-maturity to available-for-sale	0	281,100	0
Right-of-use assets obtained in exchange for operating lease liabilities	3,600	39,600	0
Transfer of loans to other real estate owned	3,300	14,100	12,100
Capitalization of internally originated mortgage servicing rights	11,700	7,300	6,000
Supplemental schedule of noncash investing activities from acquisitions:
Investment securities available for sale	0	78,700	3,100
Loans held for sale	0	500	0
Loans	0	416,600	713,100
Premises and equipment	0	24,600	14,000
Goodwill	0	75,300	101,100
Core deposit intangible	0	16,600	15,700
Company-owned life insurance	0	15,200	9,500
Interest receivable	0	2,200	3,600
Other real estate owned	0	2,400	600
Other assets	0	6,500	6,200
Total noncash assets acquired	0	638,600	866,900
Deposits	0	706,700	696,300
Securities sold under repurchase agreements	0	30,400	0
Accounts payable and accrued expenses	0	19,900	7,700
Long-term debt	0	4,100	7,000
Other borrowed funds	0	0	6,100
Trust preferred securities	0	0	4,400
Deferred tax liability	0	100	300
Total liabilities assumed	$ 0	$ (761,200)	$ (721,800)