Accumulated Other Comprehensive Income (Loss) (Details 2) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization to net income (loss) of net prior service cost (credit)		$ (0.5)	$ 1.5	$ 0.4	$ 3.0
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses		18.8	18.7	37.5	37.4
Income (loss) before income tax provision (benefit)		(41.4)	(21.6)	(173.7)	(1.0)
Income tax provision (benefit)		(25.9)	(7.7)	(60.1)	0.3
Cost of sales		(762.3)	(945.5)	(1,553.0)	(1,961.5)
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 (credit)	[1]	0.5	(1.5)	(0.4)	(3.0)
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses	[1]	(18.8)	(18.7)	(37.5)	(37.4)
Income (loss) before income tax provision (benefit)	[2]	(18.3)	(20.2)	(37.9)	(40.4)
Income tax provision (benefit)		(6.9)	(7.7)	(14.3)	(15.4)
Net income (loss)		(11.4)	(12.5)	(23.6)	(25.0)
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]	(2.9)	2.1	(7.9)	4.8
Income tax provision (benefit)		(1.1)	0.8	(3.0)	1.8
Net income (loss)		(1.8)	1.3	(4.9)	3.0
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	[3]	(6.1)	(3.9)	(12.4)	(7.5)
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	[3]		(3.9)		(7.8)
Cost of Sales and Selling and Administrative Expenses	[3]	(4.7)		(11.0)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Electricity contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[3]	0.0	0.0	0.0	(0.2)
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	[3]	$ 7.9	$ 9.9	$ 15.5	$ 20.3

[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 7.

[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] {F|ahBzfndlYmZpbGluZ3MtaHJkcmoLEgZYTUxEb2MiXlhCUkxEb2NHZW5JbmZvOmVhOTQ3Y2M0N2Q0MTQyNzBhMDQ4NmQ1YjBlMWE2OGI4fFRleHRTZWxlY3Rpb246N0IzRTVBRjUxNTZBQkI3MTZGODQ0RURCQ0JBNjAzMzkM}