COMMON STOCK AND STOCKHOLDERS' EQUITY - ADDITIONAL INFORMATION (Details) - USD ($)		1 Months Ended	3 Months Ended			6 Months Ended		12 Months Ended
	Jan. 01, 2017	Jun. 30, 2015	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Feb. 02, 2015
Schedule Of Stockholders Equity [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value			$ 1,108,000		$ 2,693,000	$ 1,800,000	$ 3,087,000
Number of options outstanding (in shares)			3,869,710			3,869,710		3,843,276
Aggregate Intrinsic Value, Exercised						$ 6,648,000
Total stock-based compensation expense			$ 3,513,000		2,383,000	$ 6,010,000	$ 4,296,000
Stock Option
Schedule Of Stockholders Equity [Line Items]
Expected term - no greater than (in years)						5 years 9 months 18 days	5 years 9 months 18 days
Unrecognized stock-based compensation expense for unvested stock option			13,182,000			$ 13,182,000
Unrecognized stock-based compensation, weighted-average period expected to be recognized (in years)						2 years 7 months 6 days
Restricted Stock Units
Schedule Of Stockholders Equity [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period						4 years
Requisite service period						4 years
Unrecognized stock-based compensation expense for unvested stock option			25,033,000			$ 25,033,000
Unrecognized stock-based compensation, weighted-average period expected to be recognized (in years)						3 years 4 months 24 days
Stock Compensation Plan [Member]
Schedule Of Stockholders Equity [Line Items]
Total stock-based compensation expense			$ 262,000		0	$ 277,000	$ 271,000
2015 Equity Incentive Plan
Schedule Of Stockholders Equity [Line Items]
Number of shares included in shares available for issuance - at least (in shares)								3,915,682
Number of shares included in shares available for issuance (as a percent)								5.00%
2015 Equity Incentive Plan | Stock Option
Schedule Of Stockholders Equity [Line Items]
Expected term - no greater than (in years)				10 years
2015 Equity Incentive Plan | Restricted Stock Units
Schedule Of Stockholders Equity [Line Items]
Number of options outstanding (in shares)			1,220,479			1,220,479
2015 Employee Stock Purchase Plan
Schedule Of Stockholders Equity [Line Items]
Common stock available for issuance (in shares)			783,136			783,136
Number of shares included in shares available for issuance percentage (in shares)								1.00%
2009 Stock Option Plan
Schedule Of Stockholders Equity [Line Items]
Available shares for issuance of grants (in shares)			0			0
Common Class B
Schedule Of Stockholders Equity [Line Items]
Common stock, shares, reclassified (in shares)		11,305,355
Common stock, shares authorized (in shares)			100,000,000			100,000,000		100,000,000
Common stock, par value (in dollars per share)			$ 0.000004			$ 0.000004		$ 0.000004
Common stock, voting rights						10 votes per share
Common Class B | 2009 Stock Option Plan
Schedule Of Stockholders Equity [Line Items]
Number of options outstanding (in shares)			2,715,500			2,715,500
Common Class A
Schedule Of Stockholders Equity [Line Items]
Common stock, shares authorized (in shares)			1,000,000,000			1,000,000,000		1,000,000,000
Common stock, par value (in dollars per share)			$ 0.000004			$ 0.000004		$ 0.000004
Common stock, voting rights						1 vote per share
Common Class A | 2015 Equity Incentive Plan
Schedule Of Stockholders Equity [Line Items]
Common stock available for issuance (in shares)			5,889,213			5,889,213
Increase in number of shares available for sale (in shares)	2,029,912
Number of options outstanding (in shares)			1,154,210			1,154,210
Common Class A | 2015 Equity Incentive Plan | Fitness Mobile Apps
Schedule Of Stockholders Equity [Line Items]
Contingent consideration payable to former employees of acquiree (in shares)									207,234
Common Class A | 2015 Employee Stock Purchase Plan
Schedule Of Stockholders Equity [Line Items]
Common stock available for issuance (in shares)			1,171,960			1,171,960
Increase in number of shares available for sale (in shares)	405,982
Offering Period						24 months
Purchase price of common stock on date of purchase, (as a percent)						85.00%
Purchase period for employee stock purchase plan						P6M
Class A and B Common Stock
Schedule Of Stockholders Equity [Line Items]
Issuance of common stock under employee stock purchase plan (in shares)						132,216
Additional Paid-In Capital
Schedule Of Stockholders Equity [Line Items]
Issuance of common stock under employee stock purchase plan						$ 1,510,000
Sales and marketing
Schedule Of Stockholders Equity [Line Items]
Total stock-based compensation expense			$ 671,000		440,000	1,177,000	1,023,000
Research and development
Schedule Of Stockholders Equity [Line Items]
Total stock-based compensation expense			980,000		470,000	1,507,000	965,000
General and administrative
Schedule Of Stockholders Equity [Line Items]
Total stock-based compensation expense			$ 1,496,000		$ 1,253,000	$ 2,699,000	$ 1,873,000