Income Taxes - Additional Information (Detail)		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2020 USD ($) State Province	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2016 USD ($)
Provision for (benefit from) income taxes		$ (26,800,000)	$ (200,000)	$ (30,500,000)
Carryback period of net operating losses		5 years
Net operating loss carryback beginning date		Dec. 31, 2017
Net operating loss carryback ending date		Jan. 01, 2021
U.S. federal income taxes at statutory rate	35.00%	21.00%	21.00%	24.50%
Operating loss carryforwards		$ 27,800,000			$ 135,300,000
Net operating loss carryforward utilized		4,300,000
Goodwill		2,490,400,000	$ 2,490,600,000	$ 2,491,800,000
Goodwill amortizable for income tax		1,250,000,000
Uncertain tax positions		$ 0
U.S. [Member]
Number of States in which Entity Operates | State		50
Canada [Member]
Number of provinces in which entity operates | Province		6