Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Cash flows from operating activities:
Net income	$ 302.1	$ 226.0	$ 257.5
Adjustments to reconcile net income to cash provided by operating activities:
Stock-based compensation expense	13.3	11.8	4.7
Net cash provided by operating activities	305.6	355.0	428.0
Cash flows from investing activities:
Net cash used in investing activities	2,311.6	1,758.8	1,253.4
Cash flows from financing activities:
Purchase and retirement of common stock	(121.9)	(1.2)	(34.0)
Dividends paid to common stockholders	(194.3)	(195.9)	(195.1)
Net cash used in financing activities	(2,204.1)	(1,795.2)	(1,973.9)
Net increase (decrease) in cash and cash equivalents	413.1	318.6	(292.5)
Cash and cash equivalents at beginning of period	896.6	578.0	870.5
Cash and cash equivalents at end of period	1,309.7	896.6	578.0
Parent Company
Cash flows from operating activities:
Net income	302.1	226.0	257.5
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries	36.7	(49.1)	83.4
Stock-based compensation expense	13.3	11.9	4.6
Other, net	(5.8)	1.4	2.7
Net cash provided by operating activities	346.3	190.2	348.2
Cash flows from investing activities:
Net cash used in investing activities	0.0	0.0	0.0
Cash flows from financing activities:
Net (decrease) increase in advances from subsidiaries	(5.0)	4.6	1.8
Proceeds from issuance of long-term debt	122.4	0.0	0.0
Repayment of long-term debt	(112.6)	0.0	0.0
Purchase and retirement of common stock	(121.9)	(1.2)	(34.0)
Dividends paid to common stockholders	(194.3)	(195.9)	(195.1)
Net cash used in financing activities	(311.4)	(192.5)	(227.3)
Net increase (decrease) in cash and cash equivalents	34.9	(2.3)	120.9
Cash and cash equivalents at beginning of period	273.4	275.7	154.8
Cash and cash equivalents at end of period	$ 308.3	$ 273.4	$ 275.7