Financial Instruments - Fair Value of Company's Financial Assets Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015		Mar. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Amortized Cost		$ 39,358		$ 29,431
Available-for-sale securities, Gross Unrealized Gains				3
Available-for-sale securities, Gross Unrealized Losses		(11)
Available-for-sale securities, Estimated Fair Value		39,347		29,434
Less amounts classified as cash equivalents, Amortized Cost		(19,803)		(13,276)
Amounts classified as marketable securities, Amortized Cost		19,555		16,155
Amounts classified as marketable securities, Gross Unrealized Gains				3
Amounts classified as marketable securities, Gross Unrealized Losses		(11)
Less amounts classified as cash equivalents, Estimated Fair Value		(19,803)		(13,276)
Amounts classified as marketable securities, Estimated Fair Value	$ 16,158	19,544	[1]	16,158
U.S. Treasury securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Amortized Cost		1,003		1,016
Available-for-sale securities, Estimated Fair Value		1,003		1,016
U.S. certificates of deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Amortized Cost		5,001		7,298
Available-for-sale securities, Estimated Fair Value		5,001		7,298
Level 1 [Member] | Money market funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Amortized Cost		17,200		8,309
Available-for-sale securities, Estimated Fair Value		17,200		8,309
Level 2 [Member] | U.S. corporate debt obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Amortized Cost		11,660		9,810
Available-for-sale securities, Gross Unrealized Gains				2
Available-for-sale securities, Gross Unrealized Losses		(11)
Available-for-sale securities, Estimated Fair Value		11,649		9,812
Level 2 [Member] | Debt securities of U.S. government agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Amortized Cost		4,494		2,998
Available-for-sale securities, Gross Unrealized Gains				1
Available-for-sale securities, Estimated Fair Value		$ 4,494		$ 2,999

[1] The condensed consolidated balance sheet as of December 31, 2015 has been derived from the audited financial statements as of that date included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2015.