Accumulated Other Comprehensive Income (Loss) (Details2) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014		Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization to net income (loss) of net prior service cost (credits)		$ 6.2	$ (0.7)		$ (15.2)
Amortization of net actuarial loss		75.0	88.1		129.0
Cost of sales		(3,659.3)	(3,844.8)		(3,790.9)
Income (loss) from continuing operations before income taxes		(478.0)	1.5		(154.8)
Income tax benefit		(112.1)	(8.7)		(63.6)
Income (loss) from continuing operations		(365.9)	10.2		(91.2)
Income (loss) from discontinued operations, net of tax		0.0	(0.6)		252.8
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]	(6.2)	0.7		15.2
Amortization of net actuarial loss	[1]	(75.0)	(88.1)		(129.0)
Income (loss) from continuing operations before income taxes	[2]	(81.2)	(87.4)		(113.8)
Income tax benefit		(31.3)	(33.6)		(43.8)
Income (loss) from continuing operations		(49.9)	(53.8)		(70.0)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Currency translation adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from continuing operations	[2]	0.0	(0.5)	[3]
Income (loss) from discontinued operations, net of tax	[2],[3]				(1.5)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from continuing operations before income taxes	[2]	18.2	3.6		(14.0)
Income tax benefit		6.9	1.4		(5.4)
Income (loss) from continuing operations		11.3	2.2		(8.6)
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]	(16.9)	(1.0)		(8.8)
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]		3.4		(3.8)
Cost of Goods Sold and Selling, General and Administrative Expenses	[4]	(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]	(0.2)	0.7		(0.3)
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]		$ 0.5		$ (1.1)
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 12.

[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] Amount in 2014 is included in other income, net, and amount in 2013 is included in discontinued operations as part of the gain on sale of the tungsten materials business (see Note 3).

[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 10).