Convertible Preferred Stock, Common Stock and Stockholders' Deficit - Additional Information (Detail) (USD $)	0 Months Ended	1 Months Ended			12 Months Ended			27 Months Ended
	Sep. 07, 2012	Nov. 30, 2014	Jul. 31, 2013	Oct. 31, 2012	Dec. 31, 2014 Vote	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013
Class of Stock [Line Items]
Reverse stock split of common stock	0.5
Preferred stock, shares authorized							10,000,000
Convertible preferred stock, shares outstanding					0
Common stock shares, issued		6,088,235	5,175,000
Price per share		$ 17.00	$ 9.50
Proceeds from issuance of common stock			$ 45,800,000	$ 70,000,000	$ 86,146,000	$ 45,980,000	$ 65,788,000
Proceeds from public offering		76,300,000
Common stock, shares outstanding					48,944,530	41,787,326		41,787,326
Number of votes of each share of common stock					1
Weighted average grant date fair value					$ 8.37	$ 4.14	$ 2.97
The total intrinsic value of stock options exercised					12,100,000	5,700,000	800,000
Cash received from exercise of common stock options					2,200,000	600,000	100,000
Compensation cost related to non-vested awards not yet recognized					17,400,000
Weighted-average recognition period of compensation cost related to non-vested awards not yet recognized					1 year 10 months 24 days
2012 Purchase Plan
Class of Stock [Line Items]
Percentage of participants eligible compensation				15.00%
Common stock shares issued under purchase plan					38,085			86,120
Common stock shares reserved for future issuance and authorized for purchase					840,071
2012 Purchase Plan | At inception of the enrollment period
Class of Stock [Line Items]
Percentage of fair market value of common stock				85.00%
2012 Purchase Plan | At applicable purchase date
Class of Stock [Line Items]
Percentage of fair market value of common stock				85.00%
Minimum
Class of Stock [Line Items]
Estimated forfeiture rate					5.00%	5.00%
Maximum
Class of Stock [Line Items]
Estimated forfeiture rate					8.00%	8.00%
Parent Company
Class of Stock [Line Items]
Common stock shares, issued		4,808,824
Isis Pharmaceuticals Inc
Class of Stock [Line Items]
Common stock shares, issued		1,279,411
Initial Public Offering
Class of Stock [Line Items]
Common stock shares, issued				11,250,000
Price per share				$ 4.00
Proceeds from issuance of common stock				39,500,000
Initial Public Offering | Convertible Note Payable
Class of Stock [Line Items]
Common stock issued upon conversion of convertible notes				2,703,269
Initial Public Offering | Convertible Preferred Stock
Class of Stock [Line Items]
Common stock issued upon conversion of convertible notes				13,699,999
Astra Zeneca
Class of Stock [Line Items]
Common stock shares, issued				6,250,000
Price per share				$ 4.00
Proceeds from issuance of common stock				25,000,000
Underwriter
Class of Stock [Line Items]
Common stock shares, issued				1,480,982
Price per share				$ 4.00
Proceeds from issuance of common stock				$ 5,500,000