Pension Plan and Retirement Benefits - Cost Assumptions (Details)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Postretirement Health Care Benefits
Assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	3.50%	4.25%	4.25%
U.S. pension plan
Assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	3.25%	4.00%	4.00%
Expected return on plan assets (as a percent)	7.50%	7.50%	7.50%
Rate of compensation increase (as a percent)	3.50%	3.50%	3.50%
U.K. pension plan
Assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	2.30%	3.70%	3.90%
Expected return on plan assets (as a percent)	2.70%	4.10%	4.40%