Balance Sheet Accounts and Supplemental Disclosures - Accumulated Other Comprehensive Loss, Net of Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 576.7	$ 421.3
Actuarial losses, net of income taxes	(31.5)	(26.5)
Amortization of net actuarial losses and prior service credits, net of income taxes	40.4	29.4
Ending balance	233.5	576.7
Actuarial Losses, Net
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(236.4)	(239.4)
Actuarial losses, net of income taxes	(31.5)	(26.5)
Amortization of net actuarial losses and prior service credits, net of income taxes	40.3	29.5
Ending balance	(227.6)	(236.4)
Prior Service Credits (Costs), Net
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(0.1)	0.0
Actuarial losses, net of income taxes	0.0	0.0
Amortization of net actuarial losses and prior service credits, net of income taxes	0.1	(0.1)
Ending balance	0.0	(0.1)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(236.5)	(239.4)
Ending balance	$ (227.6)	$ (236.5)