Fair Value Measurement - Additional Information (Detail) - Senior Notes [Member] - USD ($) $ in Thousands		Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Senior Notes Payable	[1]	$ 292,008	$ 291,049	$ 290,623
Fair Value, Inputs, Level 2 [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Senior Notes Payable		300,000
Long-term Debt, Fair Value		$ 323,600

[1] Interest rate of 6.38% as of June 30, 2017, September 30, 2016 and June 30, 2016