Senior Notes - Additional Information (Detail)	Jul. 14, 2010 USD ($) Contract	Sep. 30, 2018 USD ($)	May 08, 2018 USD ($)	Sep. 30, 2017 USD ($)	May 07, 2008 USD ($) Contract
Debt Disclosure [Line Items]
Debt Issuance Costs, Net		$ 6,100,000		$ 200,000
May 2008 Senior Notes
Debt Disclosure [Line Items]
Senior Notes issued in a private placement to a group of institutional investors					$ 275,000,000
Number of series of Senior Notes issued | Contract					4
July 2010 Senior Notes
Debt Disclosure [Line Items]
Senior Notes issued in a private placement to a group of institutional investors	$ 245,000,000
Number of series of Senior Notes issued | Contract	4
Debt Instrument, Covenant, Leverage Ratio, Maximum	3.00
Debt Instrument, Covenant, Fixed Charge Coverage Ratio, Minimum	2.50
May 2018 Senior Notes
Debt Disclosure [Line Items]
Senior Notes issued in a private placement to a group of institutional investors			$ 400,000,000