Stock Incentive Plans - Additional Information (Detail) (USD $)	12 Months Ended								1 Months Ended						12 Months Ended					12 Months Ended			1 Months Ended	12 Months Ended			1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jan. 31, 2010	Dec. 31, 2012 Regulus Therapeutics Inc	Dec. 31, 2011 Regulus Therapeutics Inc	Dec. 31, 2010 Regulus Therapeutics Inc	Dec. 31, 2012 Employee Stock Option	Jun. 30, 2009 Stock Incentive Plan 2009	Jun. 30, 2009 Stock Incentive Plan 2009 Board of Directors	Jun. 30, 2009 Stock Incentive Plan 2009 Board of Directors Initial Appointment	Jun. 30, 2009 Stock Incentive Plan 2009 Board of Directors On Date Of Annual Meeting Of Shareholders	Jun. 30, 2009 Stock Incentive Plan 2009 Chairperson of Audit Committee Stock Options Additional Annual Grant	Jun. 30, 2009 Stock Incentive Plan 2009 Chairperson of Science and Technology Committee Stock Options Additional Annual Grant	Dec. 31, 2012 Employee Stock Purchase Plan	Dec. 31, 2011 Employee Stock Purchase Plan	Dec. 31, 2010 Employee Stock Purchase Plan	Jun. 30, 2010 Employee Stock Purchase Plan	Dec. 31, 2004 Employee Stock Purchase Plan	Dec. 31, 2012 Employee Stock Option And Employee Stock Purchase Plan	Dec. 31, 2011 Employee Stock Option And Employee Stock Purchase Plan	Dec. 31, 2010 Employee Stock Option And Employee Stock Purchase Plan	Oct. 31, 2010 Unvested Restricted Common Stock	Dec. 31, 2012 Unvested Restricted Common Stock	Dec. 31, 2011 Unvested Restricted Common Stock	Dec. 31, 2010 Unvested Restricted Common Stock	Jan. 31, 2012 Unvested Restricted Common Stock Current Employees	May 31, 2011 Performance Based Restricted Stock Awards	Dec. 31, 2010 Stock Incentive Plan 2004
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards, shares authorized									2,200,000									715,789	315,789										12,366,485
Stock incentive plan, shares authorized for non-employee directors											30,000	15,000	10,000	15,000
Stock incentive plan, non-employee directors required service period										6 months
Stock incentive plan, non-employee directors award vest on each anniversary of grant date									33.333%
Stock incentive plan, shares available for future grant	815,916							541,806							274,110
Options expiration period	10 years
Option vesting percentage on the first anniversary of grant date	25.00%
Option vesting percentage at the end of each successive three-month period	6.25%
Stock-based compensation expense	$ 12,360,000	$ 16,676,000	$ 19,118,000												$ 200,000	$ 300,000	$ 300,000			$ 9,000,000	$ 14,800,000	$ 18,700,000
Non-employee stock-based compensation expenses	1,000,000	400,000		300,000
Joint venture stock-based compensation (Regulus Therapeutics Inc.)	321,000	370,000	289,000		289,000	370,000	321,000
Equity awards granted																							113,370	513,082			513,082	229,806
Fair value of equity awards granted																							1,400,000				5,300,000	2,300,000
Stock awards vesting period																							3 years				2 years
Equity awards, term																												5 years
Restricted stock expense	100,000	1,500,000	2,400,000
Total compensation cost for all stock-based awards	12,700,000	17,100,000	19,400,000
Percentage of Stock options expected to vest	67.00%
Expected annual forfeiture rate	9.50%
Unearned compensation expense related to stock options	14,700,000
Weighted-average recognition period	2 years 7 months 6 days																							1 year
Weighted average remaining contractual life for stock options outstanding	6 years 4 months 24 days
Weighted average remaining contractual life for stock options exercisable	5 years 2 months 12 days
Aggregate intrinsic value of stock option outstanding	41,800,000
Aggregate intrinsic value of stock option exercisable	24,900,000
Aggregate intrinsic value of stock option exercised	4,800,000	40,000	1,800,000
Weighted average fair value of stock options granted	$ 8.68	$ 7.68	$ 5.98
Aggregate intrinsic value of stock options expected to vest	14,700,000
Weighted average fair value of stock options expected to vest	$ 5.56
Weighted average remaining contractual life for stock options expected to vest	8 years 9 months 18 days
weighted average exercise price for these stock options expected to vest during period	$ 12.82
Total fair value of restricted stock awards vested during period																								1,500,000	500,000	0
Unrecognized compensation expense related to non-vested award																								$ 2,800,000
Employee stock purchase plan, offering period															6 months
Employee stock purchase plan, purchase price as a percentage of common stock closing price															85.00%
Employee stock purchase plan, shares issued															73,590	79,038	72,674
Weighted average fair value awards granted															$ 2.82	$ 3.46	$ 5.12							$ 10.49
Weighted-average stock-price volatility															57.00%
Expected option life															6 months
Risk-free interest rate															0.10%