Accumulated Other Comprehensive Loss (AOCL) - Changes In Components Of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended		9 Months Ended		12 Months Ended
		Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, start		$ 762.2		$ 829.6	$ 762.2	$ 829.6	$ 762.2	$ 829.6	$ 762.2	$ 829.6	$ 756.7
Impact of ASU 2018-02												$ (1.3)
Other comprehensive (loss) before reclassifications									(12.7)	(7.3)	20.3
Net amount reclassified to net income (loss)									2.9	0.3	1.9
Balance, end			$ 762.2						675.0	762.2	829.6
Cost of goods sold									1,860.5	1,916.1	1,457.8
Selling, general and administrative expenses									47.4	40.2	44.8
Other operating expense, net									(0.8)	(0.4)	(2.1)
Income tax benefit (expense)									8.4	0.2	(7.6)
Net income (loss)		(34.6)	(65.0)	(0.3)	(20.7)	19.4	(20.7)	(20.3)	(80.8)	(66.2)	48.6
Defined benefit plan and other postretirement liabilities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, start		(100.7)		(93.8)	(100.7)	(93.8)	(100.7)	(93.8)	(100.7)	(93.8)	(116.2)
Impact of ASU 2018-02												(1.3)
Other comprehensive (loss) before reclassifications									(12.7)	(7.3)	20.3
Net amount reclassified to net income (loss)									3.0	0.4	2.1
Balance, end			(100.7)						(111.7)	(100.7)	(93.8)
Unrealized gain (loss) on financial instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, start		2.0		2.1	2.0	2.1	2.0	2.1	2.0	2.1	2.3
Impact of ASU 2018-02												0.0
Other comprehensive (loss) before reclassifications									0.0	0.0	0.0
Net amount reclassified to net income (loss)									(0.1)	(0.1)	(0.2)
Balance, end			2.0						1.9	2.0	2.1
Accumulated other comprehensive loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, start		$ (98.7)		$ (91.7)	$ (98.7)	$ (91.7)	$ (98.7)	$ (91.7)	(98.7)	(91.7)	(113.9)
Impact of ASU 2018-02	[1]											$ (1.3)
Balance, end			$ (98.7)						(109.8)	(98.7)	(91.7)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Defined benefit plan and other postretirement liabilities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cost of goods sold									(3.2)	2.8	3.1
Selling, general and administrative expenses									(4.4)	(1.9)	(0.4)
Other operating expense, net									(1.0)	1.2	1.0
Income tax benefit (expense)									(1.1)	(1.6)	(1.6)
Net income (loss)									(9.7)	0.5	2.1
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized gain (loss) on financial instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cost of goods sold									(0.2)	(0.2)	(0.2)
Income tax benefit (expense)									0.1	(0.0)	(0.0)
Net income (loss)									$ (0.1)	$ (0.2)	$ (0.2)

[1] ASU 2018-02. See Note 14. Income Taxes for further information regarding our adoption of ASU 2018-02.