Balance Sheet Accounts and Supplemental Disclosures - Accumulated Other Comprehensive Loss, Net of Income Taxes (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ (341.7)	$ (303.0)	$ (347.0)
Actuarial gains arising during the period, net of income taxes	(8.6)	(8.5)	7.5
Amortization of actuarial losses and prior service credits, net of income taxes	3.3	38.9	36.5
Ending balance	(347.0)	(272.6)	(303.0)
Actuarial Losses, Net
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(342.6)	(303.2)	(347.8)
Actuarial gains arising during the period, net of income taxes	(8.6)	(8.5)	7.5
Amortization of actuarial losses and prior service credits, net of income taxes	3.4	39.0	37.1
Ending balance	(347.8)	(272.7)	(303.2)
Prior Service Credits, Net
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	0.9	0.2	0.8
Actuarial gains arising during the period, net of income taxes	0.0	0.0	0.0
Amortization of actuarial losses and prior service credits, net of income taxes	(0.1)	(0.1)	(0.6)
Ending balance	$ 0.8	$ 0.1	$ 0.2