Income Taxes (Additional Information) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Business Acquisition [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	24.53%	21.00%	24.53%
Deferred Income Tax Expense (Benefit)			$ (12,815)	$ (90,143)
Effective Income Tax Rate Reconciliation, Percent	4.40%	(1.20%)	(0.90%)	39.10%
TCJA [Domain]
Business Acquisition [Line Items]
Deferred Income Tax Expense (Benefit)				$ 87,000
Current Income Tax Expense (Benefit)				$ 2,000