Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2025	Mar. 31, 2025	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Operating Activities
Net income	$ 145.6	$ 53.2	$ 144.8	$ 66.0	$ 198.8	$ 210.8
Adjustments to reconcile net income (loss) to net cash provided (required) by operating activities
Depreciation and amortization	66.0		59.3		134.2	118.0
Deferred and noncurrent income tax charges (benefits)	1.3		(6.0)		(0.1)	(6.5)
Accretion of asset retirement obligations	0.8		0.8		1.7	1.6	$ 3.2
Amortization of discount on marketable securities					0.0	(0.1)
(Gains) losses from sale of assets	0.0		1.4		0.3	1.0
Net (increase) decrease in noncash operating working capital					37.1	55.8
Other operating activities - net					11.6	16.2
Net cash provided (required) by operating activities					383.6	396.8
Investing Activities
Property additions					(205.8)	(194.2)
Proceeds from sale of assets					1.8	1.6
Redemptions of marketable securities					0.0	4.0
Other investing activities - net					(0.3)	(0.8)
Net cash provided (required) by investing activities					(204.3)	(189.4)
Financing Activities
Purchase of treasury stock					(363.8)	(192.5)
Dividends paid					(19.6)	(17.9)
Borrowings of debt					1,782.3	120.0
Repayments of debt					(1,543.0)	(127.8)
Debt issuance costs					(8.9)	0.0
Amounts related to share-based compensation					(19.2)	(27.2)
Net cash provided (required) by financing activities					(172.2)	(245.4)
Net increase (decrease) in cash, cash equivalents, and restricted cash					7.1	(38.0)
Cash, cash equivalents, and restricted cash at beginning of period		$ 47.0		$ 117.8	47.0	117.8	117.8
Cash, cash equivalents, and restricted cash at end of period	$ 54.1		$ 79.8		$ 54.1	$ 79.8	$ 47.0