Consolidated Statements of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net income	$ 141.0	$ 114.3
Adjustments to reconcile net income to net cash provided by operating activities:
(Reversal of) provision for credit losses	(5.1)	53.7
Net loss on disposal of premises and equipment	(1.9)	0.1
Depreciation and amortization	33.4	33.0
Net premium amortization on investment securities	29.0	10.2
Net gain on investment securities transactions	(0.2)	(0.1)
Realized and unrealized net gains on mortgage banking activities	(19.1)	(39.3)
Net gain on sale of investments in unrelated entities	0.0	(1.0)
Net gain on sale of OREO	(0.2)	(0.7)
Write-downs of OREO and other assets pending disposal	0.0	0.1
Mortgage servicing rights (recovery) impairment	(5.9)	9.9
Deferred taxes	(4.6)	7.1
Net increase in cash surrender value of company-owned life insurance	(4.1)	(5.6)
Stock-based compensation expense	7.5	5.6
Originations of mortgage loans held for sale	(596.6)	(1,148.7)
Proceeds from sales of mortgage loans held for sale	645.1	1,177.2
Changes in operating assets and liabilities:
Increase in interest receivable	(1.8)	(10.0)
Increase in other assets	(21.8)	(40.3)
Increase (decrease) in accrued interest payable	0.8	(4.0)
Increase in accounts payable and accrued expenses	0.7	15.5
Net cash provided by operating activities	205.4	162.8
Payments to Acquire Held-to-maturity Securities	(1,069.5)	0.0
Purchases of investment securities:
Available-for-sale	(1,891.4)	(1,548.8)
Proceeds from sales, maturities, and pay-downs of investment securities:
Held-to-maturity	163.9	37.2
Available-for-sale	747.8	1,114.9
Proceeds from bank-owned life insurance settlements	0.0	4.4
Extensions of credit to clients, net of repayments	172.5	(1,240.0)
Recoveries of loans charged-off	7.2	5.1
Proceeds from sale of OREO	1.1	6.6
Proceeds from sale of investments in unrelated entities	0.0	2.2
Capital expenditures, net of sales	(5.5)	(17.7)
Net cash used in investing activities	(1,873.9)	(1,636.1)
Cash flows from financing activities:
Net increase in deposits	1,790.3	2,218.9
Net (decrease) increase in securities sold under repurchase agreements	(83.9)	122.7
Repayments of long-term debt	0.0	(0.1)
Advances on long-term debt	0.0	98.6
Proceeds from issuance of common stock	0.4	0.8
Purchase and retirement of common stock	(5.4)	(78.9)
Dividends paid to common stockholders	(76.2)	(104.9)
Net cash provided by financing activities	1,625.2	2,257.1
Net (decrease) increase in cash and cash equivalents	(43.3)	783.8
Cash and cash equivalents at beginning of period	2,276.8	1,076.8
Cash and cash equivalents at end of period	2,233.5	1,860.6
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	41.3	36.0
Cash paid during the period for interest expense	12.5	25.4
Supplemental disclosures of non-cash investing and financing activities:
Right-of-use assets obtained in exchange for operating lease liabilities	0.8	3.5
Net change in unamortized gains on available-for-sale securities transferred into held-to-maturity	22.3	0.0
Transfer of loans to other real estate owned	0.7	3.2
Capitalization of internally originated mortgage servicing rights	$ 2.0	$ 9.5