Marketable Securities Measured at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 91,703	$ 24,126
Unrealized gains	9	2
Unrealized loss	(14)	(3)
Fair value	91,698	24,125
U.S. Agency notes
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	20,966	3,856
Unrealized gains	3	1
Unrealized loss	(1)
Fair value	20,968	3,857
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	43,521	11,128
Unrealized gains	3	1
Unrealized loss	(13)	(3)
Fair value	43,511	11,126
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	27,216	9,142
Unrealized gains	3
Fair value	$ 27,219	$ 9,142