Goodwill and Intangible Assets - Net - Summary of Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2022	Sep. 30, 2021
Goodwill [Line Items]
Goodwill, beginning balance	$ 951,390
Acquisitions	36,050
Goodwill, ending balance	987,440
Accumulated impairment losses	32,200	$ 32,200
Residential [Member]
Goodwill [Line Items]
Goodwill, beginning balance	911,001
Acquisitions	36,050
Goodwill, ending balance	947,051
Commercial [Member]
Goodwill [Line Items]
Goodwill, beginning balance	40,389
Acquisitions	0
Goodwill, ending balance	40,389
Accumulated impairment losses	$ 32,200	$ 32,200