Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions				3 Months Ended																12 Months Ended
		Jan. 03, 2017	Oct. 01, 2016	Sep. 30, 2017		Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2016	[1]	Sep. 30, 2016		Jun. 30, 2016	[1]	Mar. 31, 2016	[1]	Dec. 31, 2015	[1]	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Pension settlement loss				$ 0.9	[1]	$ 5.5		$ 4.1		$ 0.0		$ 2.1	[1]	$ 1.0		$ 2.0		$ 0.0		$ 10.5	[1]	$ 5.1	[1]	$ 19.3
Shares of common stock in ESOP				2,483,225																2,483,225
Defined contribution plan, cost recognized																				$ 33.7		34.6		36.8
Materials Technologies
Defined Benefit Plan Disclosure [Line Items]
Periodic benefit costs including settlement, curtailment, and special termination benefits																				6.0
Materials Technologies | Business Separation Costs
Defined Benefit Plan Disclosure [Line Items]
Periodic benefit costs including settlement, curtailment, and special termination benefits																				2.5
Materials Technologies | Discontinued Operations
Defined Benefit Plan Disclosure [Line Items]
Periodic benefit costs including settlement, curtailment, and special termination benefits																				$ 3.5
U.S.
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets																				8.00%
U.S. | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets																				8.20%
U.S. | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets																				5.00%
U.S. | Real Estate/Other
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets																				7.00%
International
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets																				6.10%
U.K.
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets																				6.60%
U.K. | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets																				7.30%
U.K. | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets																				3.50%
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Pension settlement loss																				$ 10.5		5.1		19.3
Approximate average remaining service period																				9 years
ABO for all defined benefit pension plans				$ 4,842.8								4,954.9								$ 4,842.8		4,954.9
Company contributions																				64.1
Defined Benefit Pension Plan | Electronic Materials Division (EMD)
Defined Benefit Plan Disclosure [Line Items]
Decrease in underfunded status of pension plan due to transfer of pension assets and obligations			$ 24.0
Decrease in accumulated other comprehensive loss, net of tax, due to transfer pension unrecognized losses			5.0
Defined Benefit Pension Plan | Performance Materials Division (PMD)
Defined Benefit Plan Disclosure [Line Items]
Decrease in underfunded status of pension plan due to transfer of pension assets and obligations		$ 7.0
Defined Benefit Pension Plan | Minimum
Defined Benefit Plan Disclosure [Line Items]
Total expected contributions for next fiscal year				50.0																50.0
Defined Benefit Pension Plan | Maximum
Defined Benefit Plan Disclosure [Line Items]
Total expected contributions for next fiscal year				70.0																70.0
Defined Benefit Pension Plan | Unfunded Plan
Defined Benefit Plan Disclosure [Line Items]
ABO for plans with obligation in excess plan assets				99.0																99.0
PBO for plans with obligation in excess plan assets				107.8																107.8
Defined Benefit Pension Plan | U.S.
Defined Benefit Plan Disclosure [Line Items]
Pension settlement loss																				10.5		5.1		$ 18.9
Component of benefits paid related to lump sum payment to terminated participants																						52.9
ABO for plans with obligation in excess plan assets				2,951.0								3,242.5								2,951.0		3,242.5
PBO for plans with obligation in excess plan assets				$ 3,116.7								$ 3,477.7								$ 3,116.7		$ 3,477.7
Expected return on plan assets																				8.00%		8.00%		8.30%
Company contributions																				$ 27.2		$ 13.9
Net periodic postretirement cost discount rate	[2]																			3.50%		4.30%		4.30%
Accumulated postretirement benefit obligation discount rate				3.80%								3.50%								3.80%		3.50%
Net actuarial loss				$ 980.5								$ 1,273.6								$ 980.5		$ 1,273.6
Actuarial loss estimated				88.5																88.5
Periodic benefit costs including settlement, curtailment, and special termination benefits																				37.4		40.1		$ 79.0
Defined Benefit Pension Plan | International
Defined Benefit Plan Disclosure [Line Items]
Pension settlement loss																				1.7		1.3		$ 2.3
ABO for plans with obligation in excess plan assets				365.6								1,673.6								365.6		1,673.6
PBO for plans with obligation in excess plan assets				$ 465.7								$ 1,849.6								$ 465.7		$ 1,849.6
Expected return on plan assets																				6.10%		6.30%		6.10%
Company contributions																				$ 42.2		$ 65.4
Net periodic postretirement cost discount rate	[2]																			2.00%		3.30%		3.60%
Accumulated postretirement benefit obligation discount rate				2.40%								2.00%								2.40%		2.00%
Net actuarial loss				$ 551.9								$ 769.6								$ 551.9		$ 769.6
Actuarial loss estimated				$ 39.9																39.9
Periodic benefit costs including settlement, curtailment, and special termination benefits																				$ 39.4		$ 28.0		$ 56.6
Defined Benefit Pension Plan | International | Electronic Materials Division (EMD)
Defined Benefit Plan Disclosure [Line Items]
Decrease in underfunded status of pension plan due to transfer of pension assets and obligations			$ 3.0
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Net periodic postretirement cost discount rate																				1.90%		2.40%		2.60%
Accumulated postretirement benefit obligation discount rate				2.40%								1.90%								2.40%		1.90%
Net actuarial loss				$ 7.8								$ 18.7								$ 7.8		$ 18.7
Actuarial loss estimated				$ 0.3																0.3
Periodic benefit costs including settlement, curtailment, and special termination benefits																				$ 2.9		$ 4.9		$ 5.8

[1] For additional information, see Note 16, Retirement Benefits.

[2] Effective in 2016, the Company began to measure the service cost and interest cost components of pension expense by applying spot rates along the yield curve to the relevant projected cash flows, as we believe this provides a better measurement of these costs. The Company accounted for this in 2016 as a change in accounting estimate and, accordingly, accounted for it on a prospective basis. This change did not affect the measurement of the total benefit obligation. The 2017 discount rates used to measure the service cost and interest cost of our U.S. pension plans were 3.6% and 3.0%, respectively. The rates used to measure the service cost and interest cost of our major International pension plans were 2.1% and 1.8%, respectively. The previous method would have used a single discount rate for both service and interest costs.