Accumulated Other Comprehensive Income (Loss) (Details 2) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization to net income of net prior service credits		$ (0.6)	$ (0.4)	$ (1.2)	$ (0.8)
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses		19.1	18.0	38.3	35.8
Income (loss) before income tax provision (benefit)		80.5	11.3	146.0	34.4
Income tax provision (benefit)		4.9	(2.1)	9.9	(0.1)
Cost of sales		(835.8)	(755.9)	(1,666.2)	(1,497.0)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Post- retirement benefit plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization to net income of net prior service credits	[1]	0.6	0.4	1.2	0.8
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses	[1]	(19.1)	(18.0)	(38.3)	(35.8)
Income (loss) before income tax provision (benefit)	[2]	(18.5)	(17.6)	(37.1)	(35.0)
Income tax provision (benefit)	[3]	(4.5)	(6.4)	(8.8)	(13.0)
Net income (loss)		(14.0)	(11.2)	(28.3)	(22.0)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) before income tax provision (benefit)	[2]	4.7	1.4	7.7	2.3
Income tax provision (benefit)	[3]	1.2	0.6	1.9	0.9
Net income (loss)		3.5	0.8	5.8	1.4
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Nickel and other raw material contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	4.5	(1.6)	8.2	(2.5)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Natural gas contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	(0.1)	(1.1)	(0.5)	(3.4)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Foreign exchange contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	$ 0.3	$ 4.1	$ 0.0	$ 8.2

[1] Amounts are reported in nonoperating retirement benefit expense (see Note 9).

[2] For pretax items, positive amounts are income and negative amounts are expense in terms of the impact to net income. Tax effects are presented in conformity with ATI’s presentation in the consolidated statements of income.

[3] These amounts exclude the impact of any deferred tax asset valuation allowance, when applicable.

[4] Amounts related to derivatives are included in cost of goods sold in the period or periods the hedged item affects earnings (see Note 7).