Nature of Business - New Accounting Pronouncements (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2017	Mar. 31, 2017	Mar. 31, 2016	Sep. 30, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt issuance costs, net	$ 280	$ 280		$ 376
Accounting Standards Update 2015-03 | Other Noncurrent Assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt issuance costs, net				(400)
Accounting Standards Update 2015-03 | Long-term Debt
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt issuance costs, net				$ 400
New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Employee service share-based compensation, tax benefit from compensation expense	$ 3,600	$ 20,900
New Accounting Pronouncement, Early Adoption, Effect | Accounting Standard Updates 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Excess tax benefit from share-based compensation, operating activities			$ 14,000
Excess tax benefits from share-based compensation, financing activities			$ 14,000