Business Segment Information - Geographic information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Business segment financial data
Sales	$ 445,606	$ 361,869	$ 343,709	$ 334,677	$ 406,588	$ 375,240	$ 366,024	$ 313,813	$ 1,485,861	$ 1,461,665	$ 1,431,045
Long-lived assets, net	120,000				101,000				120,000	101,000	78,700
United States
Business segment financial data
Sales									867,400	827,000	765,000
Long-lived assets, net	101,000				86,300				101,000	86,300	65,800
United Kingdom
Business segment financial data
Sales									219,400	243,000	282,400
Long-lived assets, net	11,700				5,300				11,700	5,300	8,600
Canada
Business segment financial data
Sales									31,500	44,600	17,600
Australia
Business segment financial data
Sales									175,600	154,000	164,600
Middle East
Business segment financial data
Sales									64,800	71,000	67,700
Far East
Business segment financial data
Sales									66,900	57,400	55,300
Other foreign countries
Business segment financial data
Sales									60,300	64,700	78,400
Long-lived assets, net	$ 7,300				$ 9,400				$ 7,300	$ 9,400	$ 4,300