Leases - Schedule of Lease Cost Components (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Finance Lease Cost:
Amortization of right of use asset		$ 7.1	$ 3.5	$ 1.7
Interest on lease liabilities		3.1	1.0	0.5
Operating lease cost		22.7	20.8	20.5
Short-term lease cost		1.6	1.9	3.1
Variable lease cost		0.9	0.9	0.8
Sublease income		(0.3)	(0.1)	0.0
Total lease cost		35.1	28.0	26.6
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from finance leases		3.0	1.0	0.5
Operating cash flows from operating leases		20.0	21.7	20.8
Financing cash flows from finance leases		14.3	6.2	2.4
Right of use assets obtained in exchange for new finance lease liabilities		58.9	42.3	14.1
Right of use assets obtained in exchange for new operating lease liabilities	[1]	$ 4.8	$ 12.4	$ 35.9
Weighted average remaining lease term - finance leases		5 years	4 years	4 years
Weighted average remaining lease term - operating leases		5 years	6 years	6 years
Weighted average discount rate - finance leases		5.20%	6.20%	5.30%
Weighted average discount rate - operating leases		6.50%	6.90%	7.00%
Renewal Option Determined As Reasonably Certain
Cash paid for amounts included in the measurement of lease liabilities
Right of use assets obtained in exchange for new operating lease liabilities				$ 10.2

[1] (a) Several of the Company’s real property lease contracts include options to extend the lease term. The Company reassesses the likelihood of exercising renewal clauses on at least an annual basis. For the year ended December 31, 2019, ATI recorded $10.2 million of additional ROU assets and lease liabilities for likely renewals at certain leased locations.