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	Mar. 31, 2018	Dec. 31, 2017
Assets:
Cash and cash equivalents	$ 456,535	$ 758,505
Total assets	1,566,358	1,614,302
Liabilities:
Contingent consideration	2,765	2,231
Total liabilities	2,765	2,231
U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable securities	1,096,183	838,316
Certificates of Deposit [Member]
Assets:
Marketable securities	13,640	17,481
Quoted prices in active markets (Level 1) [Member]
Assets:
Cash and cash equivalents	329,285	758,505
Total assets	329,285	758,505
Significant other observable inputs (Level 2) [Member]
Assets:
Cash and cash equivalents	127,250
Total assets	1,237,073	855,797
Significant other observable inputs (Level 2) [Member] | U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable securities	1,096,183	838,316
Significant other observable inputs (Level 2) [Member] | Certificates of Deposit [Member]
Assets:
Marketable securities	13,640	17,481
Significant unobservable inputs (Level 3) [Member]
Liabilities:
Contingent consideration	2,765	2,231
Total liabilities	$ 2,765	$ 2,231