Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Operating Activities
Net income	$ 66.9	$ 168.9	$ 89.3	$ 69.2	$ 32.7	$ 5.3	$ 325.1	$ 107.2
Adjustments to reconcile net income (loss) to net cash provided by (required by) operating activities
Depreciation and amortization	40.6			37.6			119.5	113.8
Deferred and noncurrent income tax charges (credits)							1.7	6.9
Accretion of asset retirement obligations	0.6			0.6			1.7	1.6	$ 2.1
Pretax (gains) losses from sale of assets	0.0			(0.2)			(1.4)	(0.1)
Net (increase) decrease in noncash operating working capital							(2.2)	(3.7)
Loss on early debt extinguishment	0.0			14.8			0.0	14.8
Other operating activities - net							23.4	11.1
Net cash provided by (required by) operating activities							467.8	251.6
Investing Activities
Property additions							(169.4)	(153.7)
Proceeds from sale of assets							7.7	2.4
Other investing activities - net							(1.6)	(0.7)
Net cash provided by (required by) investing activities							(163.3)	(152.0)
Financing Activities
Purchase of treasury stock							(230.5)	(139.1)
Borrowings of debt							0.0	693.7
Repayments of debt							(26.1)	(573.1)
Debt issuance costs							0.0	(3.1)
Early debt extinguishment costs							0.0	(10.4)
Amounts related to share-based compensation							(10.7)	(4.4)
Net cash provided by (required by) financing activities							(267.3)	(36.4)
Net increase (decrease) in cash, cash equivalents, and restricted cash							37.2	63.2
Cash, cash equivalents, and restricted cash at beginning of period			$ 280.3			$ 184.5	280.3	184.5	184.5
Cash, cash equivalents, and restricted cash at end of period	$ 317.5			$ 247.7			$ 317.5	$ 247.7	$ 280.3