Marketable Securities - Summary of Available-for-Sale Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,500	$ 245,159
Gross Unrealized Gains	1	12
Gross Unrealized Losses		(138)
Aggregate Fair Value	4,501	245,033
U.S. Government and Government Agency Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,000	145,066
Gross Unrealized Gains	1
Gross Unrealized Losses		(80)
Aggregate Fair Value	3,001	144,986
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,500	100,093
Gross Unrealized Gains		12
Gross Unrealized Losses		(58)
Aggregate Fair Value	$ 1,500	$ 100,047