Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective Income Tax Rate Reconciliation, Percent
At U.S. federal statutory rate	35.00%	35.00%	35.00%
State taxes, net of federal effect	4.00%	1.80%	4.30%
Stock-based compensation	(1.10%)	(0.20%)	0.80%
Tax credits	8.00%	3.50%	8.50%
Orphan drug credit	(2.00%)	(0.60%)	(2.70%)
Other permanent items	(0.10%)		(0.30%)
Foreign rate differential	(5.40%)	(2.50%)	(4.10%)
In-process research and development		(19.40%)
Tax basis in intangible assets as a result of Sirna transaction		4.20%
Valuation allowance	(38.40%)	(11.80%)	(38.50%)
Effective income tax rate		10.00%	3.00%