Pension and Post-retirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Pension Plans
Pension Plan and Retirement Benefits
Service cost	$ 1,384	$ 1,570	$ 2,768	$ 3,141
Interest cost	2,625	2,592	5,231	5,141
Expected return	(3,665)	(3,531)	(7,299)	(7,003)
Amortizations	1,294	2,994	2,583	5,975
Net periodic benefit cost	1,638	3,625	3,283	7,254
U.S. pension plan
Pension Plan and Retirement Benefits
Contribution to plan			3,000
Expected future employer contribution			3,000
U.K. pension plan
Pension Plan and Retirement Benefits
Contribution to plan			421
Expected future employer contribution			382
Postretirement Health Care Benefits
Pension Plan and Retirement Benefits
Service cost	84	87	168	174
Interest cost	1,078	1,073	2,156	2,146
Amortizations	751	1,070	1,500	2,140
Net periodic benefit cost	$ 1,913	$ 2,230	3,824	$ 4,460
Contribution to plan			1,866
Expected future employer contribution			$ 3,134