Cash Cash Equivalents and Short-term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Total cash and cash equivalents, Amortized Cost	$ 77,403	$ 76,568	$ 65,586	$ 65,983
Total short-term investments, Amortized Cost	54,780	32,885
Total short-term investments, Unrealized Gains	8	9
Total short-term investments, Unrealized Losses	(139)	(20)
Total short-term investments, Estimated Fair Value	54,649	32,874
Cash, cash equivalents and short-term investments, Amortized Cost	132,183	109,453
Cash, cash equivalents and short-term investments, Unrealized Gains	8	9
Cash, cash equivalents and short-term investments, Unrealized Losses	(139)	(20)
Cash, cash equivalents and short-term investments, Estimated Fair Value	132,052	109,442
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments, Amortized Cost	21,471	13,389
Total short-term investments, Unrealized Gains	6	2
Total short-term investments, Unrealized Losses	(51)	(14)
Total short-term investments, Estimated Fair Value	21,426	13,377
U.S. agency securities
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments, Amortized Cost	21,126	11,280
Total short-term investments, Unrealized Gains	2	4
Total short-term investments, Unrealized Losses	(50)	(1)
Total short-term investments, Estimated Fair Value	21,078	11,283
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments, Amortized Cost	6,669	4,670
Total short-term investments, Unrealized Gains	0	1
Total short-term investments, Unrealized Losses	(29)	(5)
Total short-term investments, Estimated Fair Value	6,640	4,666
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Total short-term investments, Amortized Cost	5,514	3,546
Total short-term investments, Unrealized Gains	0	2
Total short-term investments, Unrealized Losses	(9)	0
Total short-term investments, Estimated Fair Value	5,505	3,548
Cash
Schedule of Available-for-sale Securities [Line Items]
Total cash and cash equivalents, Amortized Cost	45,706	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	45,706	59,568
Money market mutual funds
Schedule of Available-for-sale Securities [Line Items]
Total cash and cash equivalents, Amortized Cost	31,697	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	31,697	17,000
Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Total cash and cash equivalents, Amortized Cost	77,403	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	$ 77,403	$ 76,568