Share Based Compensation (Details)		3 Months Ended	12 Months Ended
	Oct. 01, 2016	Dec. 31, 2017	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares) | shares			0	0
Closing stock price (in dollars per share) | $ / shares			$ 36.01	$ 38.82
Total intrinsic value of stock options exercised			$ 400,000	$ 900,000
Unrecognized compensation cost related to non-vested stock options			100,000
Before-Tax Share-Based Compensation Award Cost			$ 10,100,000	$ 8,300,000	$ 5,000,000
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			200,000	700,000
Weighted average grant-date fair value (in dollars per share) | $ / shares			$ 47.23	$ 25.30
Share-based compensation costs paid				$ 0
Unrecognized compensation cost related to restricted stock units			$ 13,300,000	$ 13,900,000
Weighted average period to recognize compensation costs			1 year 10 months 25 days	2 years 6 months
Total fair value of shares paid out			$ 5,000,000	$ 1,700,000
Before-Tax Share-Based Compensation Award Cost			$ 9,500,000	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			$ 0	200,000
Director awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual Director Award			100,000
Before-Tax Share-Based Compensation Award Cost			$ 600,000	$ 600,000
Long-Term Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for grant (in shares) | shares			4,700,000
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			203,405	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			$ 50.18	$ 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			$ 45.71	$ 28.37
Air Products
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock split conversion ratio	0.5