SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)

6 Months Ended

Jun. 30, 2014

Potential Common Shares Excluded from the Calculation of Net Loss Per Common Share

The following table sets forth for each period presented the potential common shares (prior to consideration of the treasury stock method) excluded from the calculation of net loss per common share because their inclusion would be anti-dilutive, in thousands:

	At June 30,
	2014		2013
Options to purchase common stock		8,038		8,567
Unvested restricted common stock		23		604
		8,061		9,171

Fair Value Measured on Recurring Basis

Financial assets measured at fair value on a recurring basis are summarized as follows, in thousands:

Description	At June 30, 2014		Quoted Prices in Active Markets (Level 1)		Significant Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash equivalents	$	70,110	$	57,920	$	12,190	$	—
Marketable securities (fixed income):
Corporate notes		644,353		—		644,353		—
U.S. government-sponsored enterprise securities		59,638		—		59,638		—
Municipal debt securities		9,031		—		9,031		—
Commercial paper		162,148		—		162,148		—
Marketable securities (Regulus equity holdings)		49,450		49,450		—		—
Total	$	994,730	$	107,370	$	887,360	$	—
Description	At December 31, 2013		Quoted Prices in Active Markets (Level 1)		Significant Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash equivalents	$	32,571	$	32,571	$	—	$	—
Marketable securities (fixed income):
Corporate notes		228,462		—		228,462		—
U.S. government-sponsored enterprise securities		56,863		—		56,863		—
Commercial paper		11,978		—		11,978		—
Marketable securities (Regulus equity holdings)		45,452		45,452		—		—
Total	$	375,326	$	78,023	$	297,303	$	—

Summary of Fair Value Accumulated Other Comprehensive Income and Intraperiod Tax Allocation in Regulus Available-for-Sale Marketable Securities

The following tables summarize the fair value, accumulated other comprehensive income and intraperiod tax allocation regarding our investment in Regulus available-for-sale marketable securities at June 30, 2014 and 2013 and the activity for the three months ended June 30, 2014 and 2013, in thousands:

Description	At March 31, 2014			Three Months Ended June 30, 2014			Balance at June 30, 2014
Carrying value	$	12,449		$	—		$	12,449
Accumulated other comprehensive income, before tax		43,029			(6,028	)		37,001
Investment in equity securities of Regulus, as reported	$	55,478		$	(6,028	)	$	49,450
Accumulated other comprehensive income, before tax	$	43,029		$	(6,028	)	$	37,001
Intraperiod tax allocation recorded as a benefit from income taxes		(15,966	)		1,384			(14,582	)
Intraperiod tax allocation recorded as an accrued liability		(1,380	)		1,088			(292	)
Accumulated other comprehensive income, net of tax	$	25,683		$	(3,556	)	$	22,127
Description	At March 31, 2013			Three Months Ended June 30, 2013			Balance at June 30, 2013
Carrying value	$	12,449		$	—		$	12,449
Accumulated other comprehensive income, before tax		35,217			12,670			47,887
Investment in equity securities of Regulus, as reported	$	47,666		$	12,670		$	60,336
Accumulated other comprehensive income, before tax	$	35,217		$	12,670		$	47,887
Intraperiod tax allocation recorded as a benefit from income taxes		(11,134	)		(2,884	)		(14,018	)
Intraperiod tax allocation recorded as an accrued liability		(3,023	)		(2,209	)		(5,232	)
Accumulated other comprehensive income, net of tax	$	21,060		$	7,577		$	28,637

The following tables summarize the fair value, accumulated other comprehensive income and intraperiod tax allocation regarding our investment in Regulus available-for-sale marketable securities at June 30, 2014 and 2013 and the activity for the six months ended June 30, 2014 and 2013, in thousands:

Description	At December 31, 2013			Six Months Ended June 30, 2014			Balance at June 30, 2014
Carrying value	$	12,449		$	—		$	12,449
Accumulated other comprehensive income, before tax		33,003			3,998			37,001
Investment in equity securities of Regulus, as reported	$	45,452		$	3,998		$	49,450
Accumulated other comprehensive income, before tax	$	33,003		$	3,998		$	37,001
Intraperiod tax allocation recorded as a benefit from income taxes		(13,267	)		(1,315	)		(14,582	)
Intraperiod tax allocation recorded as an accrued liability		—			(292	)		(292	)
Accumulated other comprehensive income, net of tax	$	19,736		$	2,391		$	22,127

Description	At December 31, 2012			Six Months Ended June 30, 2013			Balance at June 30, 2013
Carrying value	$	12,449		$	—		$	12,449
Accumulated other comprehensive income, before tax		26,299			21,588			47,887
Investment in equity securities of Regulus, as reported	$	38,748		$	21,588		$	60,336
Accumulated other comprehensive income, before tax	$	26,299		$	21,588		$	47,887
Intraperiod tax allocation recorded as a benefit from income taxes		(10,572	)		(3,446	)		(14,018	)
Intraperiod tax allocation recorded as an accrued liability		—			(5,232	)		(5,232	)
Accumulated other comprehensive income, net of tax	$	15,727		$	12,910		$	28,637

Summary of Company's Marketable Securities Excluding Regulus

The following tables summarize our marketable securities, other than our holdings in Regulus noted above, at June 30, 2014 and December 31, 2013, in thousands:

	At June 30, 2014
Description	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Commercial paper (Due within 1 year)	$	162,170	$	4	$	(26	)	$	162,148
Corporate notes (Due within 1 year)		347,396		101		(67	)		347,430
Corporate notes (Due after 1 year through 2 years)		297,086		70		(233	)		296,923
U.S. government-sponsored enterprise securities (Due within 1 year)		15,001		6		—			15,007
U.S. government-sponsored enterprise securities (Due after 1 year through 2 years)		44,658		2		(29	)		44,631
Municipal debt securities (Due after 1 year through 2 years)		9,003		28		—			9,031
Total	$	875,314	$	211	$	(355	)	$	875,170
	At December 31, 2013
Description	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Commercial paper (Due within 1 year)	$	11,983	$	—	$	(5	)	$	11,978
Corporate notes (Due within 1 year)		154,175		33		(46	)		154,162
Corporate notes (Due after 1 year through 2 years)		74,312		23		(35	)		74,300
U.S. government-sponsored enterprise securities (Due within 1 year)		26,553		8		—			26,561
U.S. government-sponsored enterprise securities (Due after 1 year through 2 years)		30,300		2		—			30,302
Total	$	297,323	$	66	$	(86	)	$	297,303