INCOME TAXES - Changes in Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance, beginning of year	$ 13,820	$ 4,086	$ 4,311
Increase from acquisition	0	0	0
Increases for tax positions of prior years	839	5,762	475
Decreases for tax positions of prior years	(5,890)	(166)	(155)
Increases based on tax positions related to the current year	378	5,456	635
Decreases due to settlements with taxing authorities	0	0	(27)
Decreases due to lapse of statute of limitation	(1,179)	(1,318)	(1,153)
Balance, end of year	$ 7,968	$ 13,820	$ 4,086