Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 118,716	$ 151,570	$ 225,291	$ 285,720
Oil and gas
Disaggregation of Revenue [Line Items]
Revenues	50,142	51,954	90,171	111,948
Petrochemical
Disaggregation of Revenue [Line Items]
Revenues	10,196	47,337	19,361	77,763
Electric utility
Disaggregation of Revenue [Line Items]
Revenues	28,452	24,255	56,622	47,270
Traction power
Disaggregation of Revenue [Line Items]
Revenues	15,140	13,944	28,501	22,295
All others
Disaggregation of Revenue [Line Items]
Revenues	14,786	14,080	30,636	26,444
United States
Disaggregation of Revenue [Line Items]
Revenues	87,939	121,432	168,525	228,510
Canada
Disaggregation of Revenue [Line Items]
Revenues	17,734	15,407	31,825	29,861
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Revenues	10,937	8,816	19,193	18,982
Asia/Pacific
Disaggregation of Revenue [Line Items]
Revenues	2,009	5,456	4,725	7,142
Mexico, Central and South America
Disaggregation of Revenue [Line Items]
Revenues	$ 97	$ 459	$ 1,023	$ 1,225