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STOCKHOLDERS' EQUITY - ADDITIONAL INFORMATION (Details)		3 Months Ended	7 Months Ended	9 Months Ended	12 Months Ended
STOCKHOLDERS' EQUITY - ADDITIONAL INFORMATION (Details)	Jun. 17, 2015 shares	Dec. 31, 2017	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2015 shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares	Oct. 23, 2018 $ / right $ / shares shares	Mar. 07, 2018 shares	Mar. 10, 2017 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense \| $			$ 14,700,000		$ 14,700,000
Weighted average period for unrecognized compensation expense expected to be recognized (in years)					2 years 3 months 18 days
Purchase of common stock (in shares)			2,806,364	3,647,705	2,806,364	2,845,866	3,015,374
Recognized compensation expense \| $					$ 8,950,000	$ 5,478,000	$ 4,079,000
Total intrinsic value of options exercised \| $					$ 1,400,000	$ 1,400,000	$ 3,300,000
Weighted average fair value of options granted (in dollars per share) \| $ / shares					$ 4.56	$ 4.28	$ 3.29
Aggregate intrinsic value of options \| $			$ 8,700,000		$ 8,700,000	$ 15,600,000
Excess tax benefit on stock option exercises \| $						$ 0	$ 0
Preferred stock, par value per share (in dollars per share) \| $ / shares			$ 0.001		$ 0.001	$ 0.001
Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred stock, par value per share (in dollars per share) \| $ / shares								$ 0.001
Earnout Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Recognized compensation expense \| $					$ 400,000	$ 400,000	$ 300,000
Common stock trading period		10 days
Series B Junior Preferred Stock \| Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred stock, share conversion ratio								0.001
Number of preferred share purchase right for each share of common stock (in shares)								1
Preferred stock, par value per share (in dollars per share) \| $ / shares								$ 0.001
Preferred stock, purchase price per right (in dollars per right) \| $ / right								35.00
Preferred stock, redemption price per right (in dollars per share) \| $ / shares								$ 0.0001
IDchecker
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Earnout shares issued (in shares)				137,306			81,182
IDchecker \| Closing Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Recognized compensation expense \| $					$ 0	$ 1,200,000	$ 1,200,000
Common stock issued during acquisition (in shares)	712,790
Vesting period of shares received	27 months
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period for unrecognized compensation expense expected to be recognized (in years)					2 years 3 months 18 days
Options outstanding (in shares)			2,580,176	802,917	2,580,176	2,357,021	2,046,169
Recognized compensation expense \| $					$ 5,900,000	$ 4,000,000.0	$ 2,700,000
Unrecognized compensation expense \| $			$ 11,700,000		$ 11,700,000
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period for unrecognized compensation expense expected to be recognized (in years)					2 years 1 month 6 days
Options outstanding (in shares)			2,042,817		2,042,817
Recognized compensation expense \| $					$ 1,500,000	400,000
Unrecognized compensation expense \| $			$ 1,900,000		$ 1,900,000
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period for unrecognized compensation expense expected to be recognized (in years)					2 years 6 months
Unrecognized compensation expense \| $			$ 1,000,000.0		$ 1,000,000.0
2012 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)										9,500,000
Purchase of common stock (in shares)			1,776,798		1,776,798
Number of common stock reserved for future grants (in shares)			2,745,093		2,745,093
2012 Plan \| RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding (in shares)			2,137,338		2,137,338
2012 Plan \| Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding (in shares)			2,042,817		2,042,817
Prior Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase of common stock (in shares)			1,029,566		1,029,566
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)									1,000,000
Number of common stock reserved for future grants (in shares)			939,249		939,249
Shares issued under the plan (in shares)			60,751
Recognized compensation expense \| $					$ 200,000
ESPP \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Discount rate from market price offering date					15.00%
Discount rate from market price purchase date					0.15
ESPP \| Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Recognized compensation expense \| $					$ 1,400,000	$ 1,000,000.0	$ 1,300,000
Director Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock reserved for future grants (in shares)			445,733		445,733
Director Plan \| RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)										1,500,000
Purchase of common stock (in shares)			442,838		442,838