Stock-based Compensation - Summary of Stock Options Outstanding and Related Activity (Detail) - Employee Stock Option - HGI shares in Thousands	6 Months Ended
	Mar. 31, 2018 $ / shares shares
Options (in shares):
Stock options outstanding at September 30, 2017 | shares	3,976
Granted | shares	0
Exercised | shares	(1,565)
Stock options outstanding at March 31, 2018 | shares	2,411
Stock options vested and exercisable at March 31, 2018 | shares	2,154
Stock options outstanding and expected to vest | shares	2,411
Weighted Average Exercise Price (in dollars per share):
Stock options outstanding at September 30, 2017	$ 9.69
Exercised	6.45
Stock options outstanding at March 31, 2018	11.79
Stock options vested and exercisable at March 31, 2018	11.36
Stock options outstanding and expected to vest	11.79
Weighted Average Grant Date Fair Value (in dollars per share):
Stock options outstanding at September 30, 2017	3.88
Granted	0.00
Exercised	2.48
Stock options outstanding at March 31, 2018	4.78
Stock options vested and exercisable at March 31, 2018	4.64
Stock options outstanding and expected to vest	$ 4.78