Supporting Financial Information - Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Sep. 30, 2020
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	$ 65,854		$ 65,854			$ 64,412
Total accumulated amortization	(15,092)		(15,092)			(11,800)
Intangible assets, net	50,762		$ 50,762			52,612
Estimated useful live of intangible assets			10 years
Amortization expense of intangible assets	1,600	$ 1,200	$ 3,300	$ 2,400
IBA
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross					$ 17,700	16,223
Customer contracts and related customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	62,281		62,281			45,600
Total accumulated amortization	(14,264)		(14,264)			(11,150)
Customer contracts and related customer relationships | IBA
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross					16,700
Covenants not to compete
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	522		522			480
Total accumulated amortization	(238)		(238)			(212)
Covenants not to compete | IBA
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross					100
Trade name
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	3,051		3,051			2,109
Total accumulated amortization	$ (590)		$ (590)			$ (438)
Trade name | IBA
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross					$ 900