Cash Cash Equivalents and Short-term Investments (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Total cash and cash equivalents, Amortized Cost	$ 174,674	$ 76,568	$ 110,277	$ 65,983
Total short-term investments, Amortized Cost	103,481	32,885
Total short-term investments, Unrealized Gains	169	9
Total short-term investments, Unrealized Losses	(46)	(20)
Total short-term investments	103,604	32,874
Cash, cash equivalents and short-term investments, Amortized Cost	278,155	109,453
Cash, cash equivalents and short-term investments, Unrealized Gains	169	9
Cash, cash equivalents and short-term investments, Unrealized Losses	(46)	(20)
Cash, cash equivalents and short-term investments, Estimated Fair Value	278,278	109,442
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments, Amortized Cost	39,518	13,389
Total short-term investments, Unrealized Gains	57	2
Total short-term investments, Unrealized Losses	(20)	(14)
Total short-term investments	39,555	13,377
U.S. agency securities
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments, Amortized Cost	32,235	11,280
Total short-term investments, Unrealized Gains	45	4
Total short-term investments, Unrealized Losses	(6)	(1)
Total short-term investments	32,274	11,283
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments, Amortized Cost	13,968	4,670
Total short-term investments, Unrealized Gains	9	1
Total short-term investments, Unrealized Losses	(20)	(5)
Total short-term investments	13,957	4,666
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments, Amortized Cost	17,760	3,546
Total short-term investments, Unrealized Gains	58	2
Total short-term investments, Unrealized Losses	0	0
Total short-term investments	17,818	3,548
Cash
Schedule of Available-for-sale Securities [Line Items]
Total cash and cash equivalents, Amortized Cost	31,706	59,568
Total cash and cash equivalents, Unrealized Gains	0	0
Total cash and cash equivalents, Unrealized Losses	0	0
Total cash and cash equivalents, Estimated Fair Value	31,706	59,568
Money market mutual funds
Schedule of Available-for-sale Securities [Line Items]
Total cash and cash equivalents, Amortized Cost	142,968	17,000
Total cash and cash equivalents, Unrealized Gains	0	0
Total cash and cash equivalents, Unrealized Losses	0	0
Total cash and cash equivalents, Estimated Fair Value	142,968	17,000
Total cash and cash equivalents
Schedule of Available-for-sale Securities [Line Items]
Total cash and cash equivalents, Amortized Cost	174,674	76,568
Total cash and cash equivalents, Unrealized Gains	0	0
Total cash and cash equivalents, Unrealized Losses	0	0
Total cash and cash equivalents, Estimated Fair Value	$ 174,674	$ 76,568