Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Taxes [Line Items]
Operating loss carryforwards, valuation allowance	$ 1,900	$ 1,900	$ 1,900
Unrecognized tax benefits if unrecognized, the balance of uncertain tax benefit would affect the effective tax rate	4,700	$ 4,700	$ 4,700	$ 7,500
Federal corporate tax rate		21.00%	21.00%
Discrete net tax benefit in connection with initial analysis of tax act			$ 22,400
Decrease in net deferred tax liabilities excluding valuation allowance			22,000
Estimated increase to valuation allowance			500
Adjustment to deferred income tax benefit	900		21,500
AMT credits	$ 2,500		2,500
Provisional tax expense (benefit)		$ 15,706	$ (17,426)	$ 20,874
AMT Credit Carryover [Member]
Income Taxes [Line Items]
Provisional tax expense (benefit)		(100)
Domestic Tax Authority [Member] | 2027-2037 [Member]
Income Taxes [Line Items]
Net operating loss carryforwards		478,300
State and Local Jurisdiction [Member] | 2019-2038 [Member]
Income Taxes [Line Items]
Net operating loss carryforwards		$ 228,300
Minimum [Member] | Domestic Tax Authority [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration year		2027
Minimum [Member] | State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration year		2020
Maximum [Member] | Domestic Tax Authority [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration year		2037
Maximum [Member] | State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration year		2039