STOCKHOLDERS' EQUITY (Award Compensation Narrative) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) Schedule Plan $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Jul. 31, 2017 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock repurchase program, authorized amount				$ 200,000,000.0
Repurchase of common stock from employees (in shares) | shares	302,206
Repurchase of common stock from employees	$ 12,600,000		$ 482,000
Share retirements (in shares) | shares	301,106
Treasury stock, shares acquired (in shares) | shares	1,100
Number of equity incentive plans | Plan	3
Stock-based compensation	$ 100,415,000	$ 86,261,000	60,842,000
Capitalized stock-based compensation	$ 5,800,000	4,500,000	3,000,000
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Number of vesting schedules | Schedule	4
Exercisable period	10 years
Intrinsic value of options exercised	$ 28,000,000	$ 23,200,000	$ 26,200,000
Weighted average grant date fair value (in dollars per share) | $ / shares	$ 15.35	$ 10.16	$ 22.48
Unrecognized compensation costs	$ 17,300,000
Unrecognized compensation costs, period for recognition	2 years 4 months 17 days
Stock Options | End of year one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate	25.00%
Stock Options | First year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate	10.00%
Stock Options | Second year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate	20.00%
Stock Options | Third year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate	30.00%
Stock Options | Fourth year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate	40.00%
Stock Options | Monthly Basis First Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate	35.00%
Stock Options | Monthly Basis Second Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate	40.00%
Stock Options | Monthly Basis Third Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate	25.00%
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Number of vesting schedules | Schedule	3
RSUs vested but not issued in period (in shares) | shares	58,983
Unrecognized compensation costs	$ 233,100,000
Unrecognized compensation costs, period for recognition	2 years 8 months 9 days
Restricted Stock Units | End of year one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate	25.00%
Restricted Stock Units | First year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate	10.00%
Restricted Stock Units | Second year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate	20.00%
Restricted Stock Units | Third year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate	30.00%
Restricted Stock Units | 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%
Purchase price, percentage of fair market value	85.00%
Number of shares purchased (in shares) | shares	373,580	342,057	312,697
Weighted-average purchase price (in dollars per share) | $ / shares	$ 29.23	$ 26.12	$ 25.80
Stock-based compensation	$ 2,000,000	$ 1,500,000	$ 4,300,000