Stockholders' Equity - Change in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ (89.1)
Net current-period other comprehensive income (loss)	(2.6)	$ 8.2	$ (9.8)
Ending balance	(91.7)	(89.1)
Actuarial Losses
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(80.4)	(87.4)
Other comprehensive income (loss) before reclassifications	(10.6)	4.5
Amounts reclassified from accumulated other comprehensive loss	2.8	2.5
Net current-period other comprehensive income (loss)	(7.8)	7.0
Ending balance	(88.2)	(80.4)	(87.4)
Prior Service Costs
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(2.4)	(2.5)
Other comprehensive income (loss) before reclassifications	(0.3)	0.0
Amounts reclassified from accumulated other comprehensive loss	0.1	0.1
Net current-period other comprehensive income (loss)	(0.2)	0.1
Ending balance	(2.6)	(2.4)	(2.5)
Foreign Currency Translation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(7.1)	(8.9)
Other comprehensive income (loss) before reclassifications	8.4	1.8
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0
Net current-period other comprehensive income (loss)	8.4	1.8
Ending balance	1.3	(7.1)	(8.9)
Unrealized Gain (Loss) on Interest Rate Swaps
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	0.8	1.5
Other comprehensive income (loss) before reclassifications	(3.3)	0.0
Amounts reclassified from accumulated other comprehensive loss	0.3	(0.7)
Net current-period other comprehensive income (loss)	(3.0)	(0.7)
Ending balance	(2.2)	0.8	1.5
Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(89.1)	(97.3)
Other comprehensive income (loss) before reclassifications	(5.8)	6.3
Amounts reclassified from accumulated other comprehensive loss	3.2	1.9
Net current-period other comprehensive income (loss)	(2.6)	8.2	(9.8)
Ending balance	$ (91.7)	$ (89.1)	$ (97.3)

Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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