Commitments and Contingencies - Future Minimum Commitments (Detail) $ in Thousands	Dec. 31, 2015 USD ($)
Future Minimum Payments Due [Line Items]
2016	$ 4,450
2017	16,547
2018	22,051
2019	22,479
2020	22,918
2021 and thereafter	135,806
Total minimum lease payments	224,251
Other Operating Lease [Member]
Future Minimum Payments Due [Line Items]
2016	1,189
2017	127
Total minimum lease payments	1,316
150 Second Street Lease [Member]
Future Minimum Payments Due [Line Items]
2016	3,261
2017	3,359
2018	3,460
2019	3,563
2020	3,670
2021 and thereafter	7,674
Total minimum lease payments	24,987
60 Binney Street Lease [Member]
Future Minimum Payments Due [Line Items]
2017	13,061
2018	18,591
2019	18,916
2020	19,248
2021 and thereafter	128,132
Total minimum lease payments	$ 197,948