Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Accumulated Other Comprehensive Income Defined Benefit Plans Adjustment Net Of Tax Portion Attributable To Parent [Abstract]
Post-retirement benefit plans, Opening balance, Attributable to parent	$ (993.5)		$ (1,030.0)
Post-retirement benefit plans, OCI before reclassifications, Attributable to parent	0		0
Post-retirement benefit plans, Amounts reclassified from AOCI, Attributable to parent	18.3	16.7	54.8	50.0
Post-retirement benefit plans, Net current-period OCI, Attributable to parent	18.3		54.8
Post-retirement benefit plans, Closing balance, Attributable to parent	(975.2)		(975.2)
Accumulated Other Comprehensive Income Foreign Currency Transaction And Translation Adjustment Net Of Tax Portion Attributable To Parent [Abstract]
Currency translation adjustment, Opening balance, Attributable to parent	(7.5)		3.4
Currency translation adjustment, OCI before reclassification, Attributable to parent	12.3		1.4
Currency translation adjustment, Amounts reclassified from AOCI, Attributable to parent	0		0
Currency translation adjustment, Net current-period OCI, Attributable to parent	12.3		1.4
Currency translation adjustment, Closing balance, Attributable to parent	4.8		4.8
Accumulated Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax Attributable to Parent [Abstract]
Unrealized holding gains on securities, Opening Balance, Attributable to parent	(0.1)		(0.1)
Unrealized holding gains on securities, OCI before reclassifications, Attributable to parent	0.1		0.1
Unrealized holding gains on securities, Amounts reclassified from AOCI, Attributable to parent	0		0
Unrealized holding gains on securities, Net current-period OCI, Attributable to parent	0.1		0.1
Unrealized holding gains on securities, Closing Balance, Attributable to parent	0		0
AOCI Derivative Financial Instruments Attributable to Parent [Abstract]
Derivatives, Opening balance, Attributable to parent	(3.7)		(2.7)
Derivatives, OCI before reclassifications, Attributable to parent	(11.1)		(12.9)
Derivatives, Amounts reclassified from AOCI, Attributable to parent	2.6		3.4
Derivatives, Net current-period OCI,, Attributable to parent	(8.5)		(9.5)
Derivatives, Closing balance, Attributable to parent	(12.2)		(12.2)
Accumulated Other Comprehensive Income Loss Attributable To ATI [Abstract]
Total, Opening balance, Attributable to parent	(1,004.8)		(1,029.4)
Total, OCI before reclassifications, Attributable to parent	1.3		(11.4)
Total, Amounts reclassified from AOCI, Attributable to parent	20.9		58.2
Total, Net current-period OCI, Attributable to parent	22.2		46.8
Total, Closing balance, Attributable to parent	(982.6)		(982.6)
Accumulated Other Comprehensive Income Defined Benefit Plans Adjustment Net Of Tax Portion Attributable To Noncontrolling Interest [Abstract]
Post-retirement benefit plans, Opening balance, Attributable to noncontrolling interest	0		0
Post-retirement benefit plans, OCI before reclassifications, Attributable to noncontrolling interest	0		0
Post-retirement benefit plans, Amounts reclassified from AOCI, Attributable to noncontrolling interest			0
Post-retirement benefit plans, Net current-period OCI, Attributable to noncontrolling interest	0		0
Post-retirement benefit plans, Closing balance, Attributable to noncontrolling interest	0		0
Accumulated Other Comprehensive Income Foreign Currency Transaction And Translation Adjustment Net Of Tax Portion Attributable To Noncontrolling Interest [Abstract]
Currency translation adjustment, Opening balance, Attributable to noncontrolling interests	26.1		23.7
Currency translation adjustment, OCI before reclassifications, Attributable to noncontrolling interests	0.1		2.5
Currency translation adjustment, Amounts reclassified from AOCI, Attributable to noncontrolling interest			0
Foreign Currency adjustment, Net current-period OCI, Attributable to noncontrolling interests	0.1		2.5
Currency translation adjustment, Closing balance, Attributable to noncontrolling interests	26.2		26.2
Accumulated Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax Attributable to Noncontrolling Interest [Abstract]
Unrealized holding gains on securities, Opening balance, Attributable to noncontrolling interests	0		0
Unrealized holding gains on securities, OCI before reclassifications, Attributable to noncontrolling interests	0		0
Unrealized holding gains on securities, Amounts reclassified from AOCI, Attributable to noncontrolling interest			0
Unrealized holding gains on securities, Net current-period OCI, Attributable to noncontrolling interests	0		0
Unrealized holding gains on securities, Closing balance, Attributable to noncontrolling interests	0		0
AOCI Derivative Financial Instruments Attributable to Noncontrolling Interest [Abstract]
Derivatives, Opening balance, Attributable to noncontrolling interests	0		0
Derivatives, OCI before reclassifications, Attributable to noncontrolling interests	0		0
Derivatives, Amounts reclassified from AOCI, Attributable to Noncontrolling Interest			0
Derivatives, OCI before reclassifications, Attributable to noncontrolling interests	0		0
Derivatives, Closing balance, Attributable to noncontrolling interests	0		0
Accumulated Other Comprehensive Income Loss Attributable To Noncontrolling interest [Roll Forward]
Total, Opening balance, Attributable to Noncontrolling Interest	26.1		23.7
Total, OCI before reclassifications, Attributable to noncontrolling interest	0.1		2.5
Total, Amounts reclassified from AOCI, Attributable To Noncontrolling Interest			0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	0.1		2.5
Total, Closing balance, Attributable to Noncontrolling Interest	$ 26.2		$ 26.2