SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Numerator:
Net income (loss)	$ 57,088	$ 7,724	$ 48,928	$ 5,976
Denominator:
Weighted-average common shares outstanding (in shares)	49,313,106	48,592,506	49,304,301	48,639,339
Weighted-average unvested restricted common shares subject to forfeiture (in shares)	(32,955)	(17,088)	(40,469)	(14,813)
Shares used in calculating basic net (income) loss per common share (in shares)	49,280,151	48,575,418	49,263,832	48,624,626
Basic net income (loss) per common share (in dollars per share)	$ 1.16	$ 0.16	$ 0.99	$ 0.12
Numerator:
Net income (loss)	$ 57,088	$ 7,724	$ 48,928	$ 5,976
Denominator:
Weighted-average common shares outstanding (in shares)	49,313,106	48,592,506	49,304,301	48,639,339
Weighted-average unvested restricted common shares subject to forfeiture (in shares)	(32,955)	(17,088)	(40,469)	(14,813)
Incremental shares from assumed conversion of stock compensation plans (in shares)	303,226	596,794	301,086	670,810
Shares used in calculating diluted net income (loss) per common share (in shares)	49,583,377	49,172,212	49,564,918	49,295,336
Diluted net income (loss) per common share (in dollars per share)	$ 1.15	$ 0.16	$ 0.99	$ 0.12
Stock options
Anti-dilutive common shares not included in diluted net income per common share
Anti-dilutive common shares not included in diluted net income per common share	6,617,868		6,390,070
Restricted stock units
Anti-dilutive common shares not included in diluted net income per common share
Anti-dilutive common shares not included in diluted net income per common share	6,617,868		6,390,070
2018 Convertible Notes
Anti-dilutive common shares not included in diluted net income per common share
Stated interest rate of debt issued (as a percent)	1.50%		1.50%