Statement of Consolidated Comprehensive Income (Loss) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ (53.5)	$ (104.0)	$ 20.2
Other Comprehensive (Income) Loss, Defined Benefit Plan, before Reclassification Adjustment, after Tax [Abstract]
Net actuarial gains (losses) and other adjustments	204.6	(51.7)	1.0
Settlement adjustment	2.7	1.8	10.9
Amortization of prior net losses	12.7	12.9	14.6
Amortization of prior net service credit	(0.4)	(0.4)	(0.4)
Changes in foreign currency translation	0.9	0.4	(2.6)
Other comprehensive income (loss)	220.5	(37.0)	23.5
Comprehensive income (loss)	$ 167.0	$ (141.0)	$ 43.7