Accumulated Other Comprehensive Income (Loss) (Details2) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization to net income (loss) of net prior service cost (credits)		$ (1.6)	$ (0.4)	$ 6.2
Amortization of net actuarial loss		71.6	75.0	75.0
Cost of sales		(3,076.1)	(2,972.1)	(3,659.3)
Income (loss) before income taxes		(86.5)	(734.0)	(478.0)
Income tax benefit		(6.8)	(106.9)	(112.1)
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 (loss) of net prior service cost (credits)	[1]	1.6	0.4	(6.2)
Amortization of net actuarial loss	[1]	(71.6)	(75.0)	(75.0)
Income (loss) before income taxes	[2]	(70.0)	(74.6)	(81.2)
Income tax benefit	[3]	(26.5)	(28.3)	(31.3)
Income (loss)		(43.5)	(46.3)	(49.9)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) before income taxes	[2]	7.2	(7.9)	18.2
Income tax benefit	[3]	2.7	(3.0)	6.9
Income (loss)		4.5	(4.9)	11.3
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Nickel and other raw material contracts [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	(3.4)	(19.5)	(16.9)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Natural gas contracts [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	(5.3)
Cost of Goods Sold and Selling, General and Administrative Expenses	[4]		(14.8)	(18.2)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Electricity contracts [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	0.0	0.0	(0.2)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Foreign Exchange Contract [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	$ 15.9	$ 26.4
Cost of Goods Sold and Selling, General and Administrative Expenses	[4]			$ 53.5

[1] Amounts are included in the computation of pension and other postretirement benefit expense, which is reported in both cost of goods sold and selling and administrative expenses. For additional information, see Note 11.

[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 operations.

[3] These amounts exclude the impact of any deferred tax asset valuation allowances (see Note 14 for further explanation).

[4] Amounts related to the effective portion of the derivatives are included in cost of goods sold in the period or periods the hedged item affects earnings. Amounts related to the ineffective portion of the derivatives are presented in selling and administrative expenses on the consolidated statements of operations (see Note 9).