Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Cash Flows from Operating Activities:
Net (loss) income	$ (1.4)	$ (163.1)	$ 12.3	$ (147.6)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Long-lived asset and goodwill impairment	0.0	247.4	0.0	247.4
Depreciation and amortization expense	33.5	35.6	101.7	109.8
Deferred income tax expense (benefit)			15.4	(64.2)
Payments in excess of expense for postretirement plan benefits			(1.4)	(1.5)
Share-based compensation expense			2.9	3.3
Gain on extinguishment of debt	(0.5)	(1.5)	(3.4)	(1.5)
Changes in working capital pertaining to operating activities:
Receivables			12.7	12.7
Inventories			17.3	(46.6)
Accounts payable			(38.8)	6.0
Accrued liabilities			(3.3)	(2.2)
Interest payable			11.7	10.8
Income taxes			(5.2)	(2.4)
Other			1.2	(3.5)
Net cash provided by operating activities			123.1	120.5
Cash Flows from Investing Activities:
Capital expenditures	(16.5)	(28.4)	(53.4)	(81.5)
Other investing activities			(1.4)	0.2
Net cash used in investing activities			(54.8)	(81.3)
Cash Flows from Financing Activities:
Repayment of long-term debt			(15.8)	(90.5)
Debt issuance costs			0.0	(2.0)
Proceeds from revolving credit facility			407.9	392.6
Repayment of revolving credit facility			(446.9)	(354.3)
Repayment of financing obligation			(2.1)	(2.1)
Dividends paid			(15.0)	0.0
Shares repurchased			(7.0)	(13.2)
Cash distribution to noncontrolling interests			0.0	(14.2)
Other financing activities			(0.5)	(7.5)
Net cash used in financing activities			(79.4)	(91.2)
Net decrease in cash and cash equivalents			(11.1)	(52.0)
Cash and cash equivalents at beginning of period			97.1	145.7	$ 145.7
Cash and cash equivalents at end of period	$ 86.0	$ 93.7	86.0	93.7	$ 97.1
Supplemental Disclosure of Cash Flow Information
Interest paid, net of capitalized interest of $0.1 million and $2.3 million, respectively			28.0	32.3
Income taxes paid, net of refunds of $0.3 million and zero, respectively			$ 2.6	$ 8.8