Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net income	$ 192.1	$ 161.2	$ 181.0
Adjustments to reconcile net income to cash provided by operating activities:
Stock-based compensation expense	8.9	7.5	8.0
Net cash provided by operating activities	282.3	268.3	127.3
Cash flows from investing activities:
Acquisition of banks and bank holding companies, net of cash and cash equivalents received	0.0	0.0	(298.4)
Net cash used in investing activities	(2,120.0)	(1,869.8)	(16.8)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of stock issuance costs	0.4	1.1	1.0
Purchase and retirement of common stock	(5.4)	(116.8)	(2.5)
Dividends paid to common stockholders	(101.6)	(128.6)	(79.2)
Net cash used in financing activities	1,905.7	2,801.5	144.3
Net increase in cash and cash equivalents	68.0	1,200.0	254.8
Cash and cash equivalents at beginning of period	2,276.8	1,076.8	822.0
Cash and cash equivalents at end of period	2,344.8	2,276.8	1,076.8
Parent Company
Cash flows from operating activities:
Net income	192.1	161.2	181.0
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries	(57.0)	(50.0)	(35.6)
Stock-based compensation expense	8.9	7.5	8.0
Other, net	(3.2)	(13.6)	8.1
Net cash provided by operating activities	140.8	105.1	161.5
Cash flows from financing activities:
Net (decrease) increase in advances from subsidiaries	23.7	16.7	(6.6)
Proceeds from issuance of long-term debt	0.0	98.6	0.0
Proceeds from issuance of common stock, net of stock issuance costs	0.4	1.1	1.0
Purchase and retirement of common stock	(5.4)	(116.8)	(2.5)
Dividends paid to common stockholders	(101.6)	(128.6)	(79.2)
Net cash used in financing activities	(82.9)	(129.0)	(87.3)
Net increase in cash and cash equivalents	57.9	(23.9)	74.2
Cash and cash equivalents at beginning of period	123.2	147.1	72.9
Cash and cash equivalents at end of period	$ 181.1	$ 123.2	147.1
BOTC Acquisition
Cash flows from financing activities:
Stock Issued			$ 176.1