Consolidated Statements of Cash Flows (unaudited) - USD ($) $ in Millions	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Operating Activities
Net income	$ 183.3	$ 152.4	$ 128.8	$ 55.3	$ 335.7	$ 184.1
Adjustments to reconcile net income (loss) to net cash provided by (required by) operating activities
Depreciation and amortization	54.7		53.3		110.1	104.3
Deferred and noncurrent income tax charges (credits)					13.1	11.7
Accretion of asset retirement obligations	0.7		0.7		1.4	1.3	$ 2.5
(Gains) losses from sale of assets	(1.9)		0.1		(1.9)	(0.1)
Net (increase) decrease in noncash operating working capital					47.8	17.3
Other operating activities - net					10.3	12.2
Net cash provided by (required by) operating activities					516.5	330.8
Investing Activities
Property additions					(144.9)	(136.8)
Payments for acquisition, net of cash acquired					0.0	(641.1)
Proceeds from sale of assets					8.1	0.8
Other investing activities - net					(0.6)	(1.2)
Net cash provided by (required by) investing activities					(137.4)	(778.3)
Financing Activities
Purchase of treasury stock					(355.4)	(198.3)
Dividends paid					(14.6)	(13.5)
Borrowings of debt					0.0	892.8
Repayments of debt					(7.6)	(216.9)
Debt issuance costs					0.0	(8.9)
Amounts related to share-based compensation					(17.5)	(6.3)
Net cash provided by (required by) financing activities					(395.1)	448.9
Net increase (decrease) in cash, cash equivalents, and restricted cash					(16.0)	1.4
Cash, cash equivalents, and restricted cash at beginning of period		$ 256.4		$ 163.6	256.4	163.6	163.6
Cash, cash equivalents, and restricted cash at end of period	$ 240.4		$ 165.0		$ 240.4	$ 165.0	$ 256.4