Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets:
Cash and cash equivalents	$ 237,962	$ 278,887
Total assets	1,142,630	884,830
Liabilities:
Contingent consideration	2,961	7,756
Total liabilities	2,961	7,756
U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable securities	886,186	599,460
Certificates of Deposit [Member]
Assets:
Marketable securities	18,482	6,483
Quoted prices in active markets (Level 1) [Member]
Assets:
Cash and cash equivalents	227,245	278,887
Total assets	227,245	278,887
Significant other observable inputs (Level 2) [Member]
Assets:
Cash and cash equivalents	10,717
Total assets	915,385	605,943
Significant other observable inputs (Level 2) [Member] | U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable securities	886,186	599,460
Significant other observable inputs (Level 2) [Member] | Certificates of Deposit [Member]
Assets:
Marketable securities	18,482	6,483
Significant unobservable inputs (Level 3) [Member]
Liabilities:
Contingent consideration	2,961	7,756
Total liabilities	$ 2,961	$ 7,756