Restructuring Charges (Details) $ in Millions	3 Months Ended					9 Months Ended		12 Months Ended
	Dec. 31, 2016 USD ($) employee	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($) employee	Mar. 31, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2016 USD ($) employee
Restructuring Cost and Reserve [Line Items]
Restructuring charges		$ 488.6			$ 0.0	$ 498.6	$ 0.0
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning of period	$ 23.2			$ 8.1		8.1		$ 8.1
Restructuring Charges						27.0
Payments for Restructuring						(11.9)
Restructuring reserve, end of period		23.2				23.2
Long-lived impairment charges
Restructuring Cost and Reserve [Line Items]
Restructuring charges		471.3
Facility shutdown and idling costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges		16.3
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning of period	16.3			3.6		3.6		3.6
Restructuring Charges						16.0
Payments for Restructuring						(3.3)
Restructuring reserve, end of period		16.3				16.3
Employee benefit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges		1.0
Employee severance
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 1.0	9.0
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning of period	$ 6.9			$ 4.5		4.5		$ 4.5
Restructuring Charges						11.0
Payments for Restructuring						(8.6)
Restructuring reserve, end of period		6.9				6.9
Rowley, UT facility
Restructuring Cost and Reserve [Line Items]
Non-cash impairment charge		470.8
Estimated fair value after impairment		$ 15.0				$ 15.0
Rowley, UT facility | Forecast
Restructuring Cost and Reserve [Line Items]
Number of positions expected to be eliminated | employee	150
Flat Rolled Products | Forecast
Restructuring Cost and Reserve [Line Items]
Number of positions expected to be eliminated | employee								250
High Performance Materials & Components | Employee severance
Restructuring Cost and Reserve [Line Items]
Number of positions eliminated | employee			20