Fair Value Measurements and Fair Value of Financial Instruments - Financial Assets Measured and Recognized at Fair Value on Recurring Basis (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Assets:
Total assets	$ 259,234,000	$ 40,000,000
Money Market Funds [Member]
Assets:
Total assets	191,024,000	40,000,000
Certificates of Deposit [Member]
Assets:
Total assets	4,082,000
U.S. Treasury Securities [Member]
Assets:
Total assets	25,167,000
Quoted Prices in Active Markets (Level 1) [Member]
Assets:
Total assets	191,024,000	40,000,000
Quoted Prices in Active Markets (Level 1) [Member] | Money Market Funds [Member]
Assets:
Total assets	191,024,000	40,000,000
Significant Other Observable Inputs (Level 2) [Member]
Assets:
Total assets	68,210,000
Significant Other Observable Inputs (Level 2) [Member] | Certificates of Deposit [Member]
Assets:
Total assets	4,082,000
Significant Other Observable Inputs (Level 2) [Member] | U.S. Treasury Securities [Member]
Assets:
Total assets	25,167,000
Significant Unobservable Inputs (Level 3) [Member]
Assets:
Total assets	0	$ 0
U.S. Government Agency Securities [Member]
Assets:
Total assets	38,961,000
U.S. Government Agency Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets:
Total assets	$ 38,961,000