STOCKHOLDERS' EQUITY (Award Compensation Narrative) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) schedule plan $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares		Jul. 31, 2017 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock repurchase program, authorized amount						$ 200,000,000
Stock repurchased and retired during period (in shares) | shares			1,752,895
Repurchase of common stock			$ 71,993,000	$ 7,743,000	[1]
Number of equity incentive plans | plan			3
Stock-based compensation	$ 29,194,000	$ 25,309,000	$ 85,732,000	75,007,000
Capitalized stock-based compensation expense	1,900,000	1,500,000	$ 5,900,000	4,600,000
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of vesting schedules | schedule			4
Exercisable period			10 years
Intrinsic value of options exercised	$ 5,300,000	$ 9,800,000	$ 18,100,000	$ 20,200,000
Weighted average grant date fair value (in USD per share) | $ / shares	$ 23.41	$ 13.31	$ 18.89	$ 15.35
Unrecognized compensation costs	$ 21,000,000		$ 21,000,000
Unrecognized compensation costs, period for recognition			2 years 6 months 1 day
Employee Stock Option | End of year one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			25.00%
Employee Stock Option | First year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			10.00%
Employee Stock Option | Second year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			20.00%
Employee Stock Option | Third year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			30.00%
Employee Stock Option | Fourth year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			40.00%
Employee Stock Option | Monthly Basis First Year Member
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			35.00%
Employee Stock Option | Monthly Basis Second Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			40.00%
Employee Stock Option | Monthly Basis Third Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			25.00%
Employee Stock Option | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			3 years
Employee Stock Option | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			4 years
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			4 years
Number of vesting schedules | schedule			3
Unrecognized compensation costs	$ 242,000,000		$ 242,000,000
Unrecognized compensation costs, period for recognition			2 years 7 months
RSUs | End of year one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			25.00%
RSUs | First year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			10.00%
RSUs | Second year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			20.00%
RSUs | Third year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			30.00%
RSUs | Fourth year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			40.00%
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Subscription rate of eligible compensation	15.00%		15.00%
Purchase price, percentage of fair market value			85.00%
Number of shares purchased (in shares) | shares	0	0	195,987	228,299
Weighted-average purchase price (in dollars per share) | $ / shares	$ 36.42	$ 23.73	$ 36.42	$ 23.73
Stock-based compensation	$ 700,000	$ 500,000	$ 2,000,000	$ 1,500,000

[1] As of January 1, 2018, the Company adopted ASC 606 using the full retrospective method. Accordingly, the Company has recast certain amounts in the prior period presented. Also as of January 1, 2018, the Company adopted Accounting Standards Update No. 2016-18, "Statement of Cash Flows (Subtopic 230): Restricted Cash," and recast the prior period presented. See Note 1 below for additional discussion.