Basis of presentation, principles of consolidation and significant accounting policies - Statement of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 952,137	$ 758,505	$ 237,962
Restricted cash included in receivables and other current assets	100
Restricted cash included in restricted cash and other non-current assets	13,763		13,762
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 966,000	$ 772,268	$ 251,724	$ 293,277