Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets:
Assets, fair value disclosure, recurring	$ 884,830	$ 865,763
Liabilities:
Liabilities, fair value disclosure, recurring	7,756	8,665
Contingent Consideration [Member]
Liabilities:
Liabilities, fair value disclosure, recurring	7,756	8,665
Cash and Cash Equivalents [Member]
Assets:
Assets, fair value disclosure, recurring	278,887	164,269
Certificates of Deposit [Member]
Assets:
Assets, fair value disclosure, recurring	6,483	14,347
U.S. Government Agency Securities [Member]
Assets:
Assets, fair value disclosure, recurring		687,147
U.S. Government Agency Securities and Treasuries [Member]
Assets:
Assets, fair value disclosure, recurring	599,460
Quoted prices in active markets (Level 1) [Member]
Assets:
Assets, fair value disclosure, recurring	278,887	158,269
Quoted prices in active markets (Level 1) [Member] | Cash and Cash Equivalents [Member]
Assets:
Assets, fair value disclosure, recurring	278,887	158,269
Significant other observable inputs (Level 2) [Member]
Assets:
Assets, fair value disclosure, recurring	605,943	707,494
Significant other observable inputs (Level 2) [Member] | Cash and Cash Equivalents [Member]
Assets:
Assets, fair value disclosure, recurring		6,000
Significant other observable inputs (Level 2) [Member] | Certificates of Deposit [Member]
Assets:
Assets, fair value disclosure, recurring	6,483	14,347
Significant other observable inputs (Level 2) [Member] | U.S. Government Agency Securities [Member]
Assets:
Assets, fair value disclosure, recurring		687,147
Significant other observable inputs (Level 2) [Member] | U.S. Government Agency Securities and Treasuries [Member]
Assets:
Assets, fair value disclosure, recurring	599,460
Significant unobservable inputs (Level 3) [Member]
Liabilities:
Liabilities, fair value disclosure, recurring	7,756	8,665
Significant unobservable inputs (Level 3) [Member] | Contingent Consideration [Member]
Liabilities:
Liabilities, fair value disclosure, recurring	$ 7,756	$ 8,665