Commitments and contingencies - Schedule of Minimum Lease Payments Under Non-cancelable Operating Leases (Parenthetical) (Detail)	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Exchange rate from Canadian dollars to U.S. dollars	0.7448

Foreign exchange rate used to translate amounts denominated in functional currency to reporting currency.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 30
-Section S99
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6897108&loc=SL6897125-166521

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