Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (1,559.6)	$ 270.1	$ 236.7
Currency translation adjustment
Unrealized net change arising during the period	32.5	(4.0)	(26.6)
Derivatives
Net derivatives gain (loss) on hedge transactions	(2.8)	9.7	(6.4)
Reclassification to net income (loss) of net realized (gain) loss	6.0	(4.1)	(11.7)
Income taxes on derivative transactions	0.0	(4.7)	0.0
Total	3.2	10.3	(18.1)
Postretirement benefit plans- Actuarial loss
Amortization of net actuarial loss	85.3	87.2	76.5
Net loss arising during the period	(128.4)	(180.5)	(141.4)
Postretirement benefit plans- Prior Service Cost
Amortization to net income (loss) of net prior service cost (credits)	(3.1)	(2.6)	(2.6)
Income taxes on postretirement benefit plans	0.0	(20.4)	0.0
Total	(46.2)	(75.5)	(67.5)
Other comprehensive loss, net of tax	(10.5)	(69.2)	(112.2)
Comprehensive income (loss)	(1,570.1)	200.9	124.5
Less: Comprehensive income attributable to noncontrolling interests	24.4	11.2	8.1
Comprehensive income (loss) attributable to ATI	$ (1,594.5)	$ 189.7	$ 116.4