Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Assets:
Assets, fair value disclosure, recurring	$ 492,003
Liabilities:
Liabilities, fair value disclosure, recurring	6,796
Cash and Cash Equivalents [Member]
Assets:
Assets, fair value disclosure, recurring	347,845	206,279
Contingent Consideration [Member]
Liabilities:
Liabilities, fair value disclosure, recurring	6,796
Certificates of Deposit [Member]
Assets:
Assets, fair value disclosure, recurring	12,622
U.S. Government Agency Securities [Member]
Assets:
Assets, fair value disclosure, recurring	131,536
Quoted prices in active markets (Level 1) [Member]
Assets:
Assets, fair value disclosure, recurring	347,845
Quoted prices in active markets (Level 1) [Member] | Cash and Cash Equivalents [Member]
Assets:
Assets, fair value disclosure, recurring	347,845	206,279
Significant other observable inputs (Level 2) [Member]
Assets:
Assets, fair value disclosure, recurring	144,158
Significant other observable inputs (Level 2) [Member] | Certificates of Deposit [Member]
Assets:
Assets, fair value disclosure, recurring	12,622
Significant other observable inputs (Level 2) [Member] | U.S. Government Agency Securities [Member]
Assets:
Assets, fair value disclosure, recurring	131,536
Significant unobservable inputs (Level 3) [Member]
Liabilities:
Liabilities, fair value disclosure, recurring	6,796
Significant unobservable inputs (Level 3) [Member] | Contingent Consideration [Member]
Liabilities:
Liabilities, fair value disclosure, recurring	$ 6,796