Share Based Compensation (Details)		3 Months Ended		12 Months Ended
	Oct. 01, 2016	Jun. 30, 2017 USD ($)	Dec. 31, 2016 $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares) | shares				0
Closing stock price (in dollars per share) | $ / shares				$ 38.82
Total intrinsic value of stock options exercised				$ 900,000
Unrecognized compensation cost related to non-vested stock options				100,000
Before-Tax Share-Based Compensation Award Cost				$ 8,300,000	$ 5,000,000	$ 4,700,000
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period				3 years
Vesting period				4 years
Shares granted (in shares) | shares				700,000
Weighted average grant-date fair value (in dollars per share) | $ / shares				$ 25.30
Share-based compensation costs paid		$ 0		$ 0
Unrecognized compensation cost related to restricted stock units				$ 13,900,000
Weighted average period to recognize compensation costs				2 years 6 months
Total fair value of shares paid out				$ 1,700,000
Before-Tax Share-Based Compensation Award Cost				$ 7,500,000
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				3 years
Weighted average period to recognize compensation costs				3 months
Exercise period				10 years
Before-Tax Share-Based Compensation Award Cost				$ 200,000
Director awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Before-Tax Share-Based Compensation Award Cost				600,000
Director award per quarter				$ 25,000,000
Long-Term Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for grant (in shares) | shares				3,300,000
Long-Term Incentive Plan | Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares) | shares			424,247
Weighted average grant-date fair value (in dollars per share) | $ / shares			$ 22.53
Long-Term Incentive Plan | Restricted stock units | Vesting period 1, October 1, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage (as percent)			33.33%
Long-Term Incentive Plan | Restricted stock units | Vesting period 2, October 1, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage (as percent)			33.33%
Long-Term Incentive Plan | Restricted stock units | Vesting period 3, October 1, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage (as percent)			33.33%
Long-Term Incentive Plan | Market-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period			3 years
Shares granted (in shares) | shares			304,520
Payout determination period			90 days
Long-Term Incentive Plan | Performance-based Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value (in dollars per share) | $ / shares			$ 29.68
Long-Term Incentive Plan | Performance-based Market Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value (in dollars per share) | $ / shares			$ 28.37
Air Products
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock split conversion ratio	0.5