Description of the business - Additional Information (Detail) $ in Billions	9 Months Ended
Sep. 30, 2018 USD ($)
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Date of incorporation	Apr. 16, 1992
Cash, cash equivalents and marketable securities	$ 2.0

Sum of investments and unrestricted cash as of the balance sheet date.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1,2)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Name:	us-gaap_InvestmentsAndCash
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant