Commitments and contingencies - Summary of Other Litigation Matters (Detail) (Pending Litigation [Member])	9 Months Ended
	Sep. 30, 2014
Glowczenski [Member]
Loss Contingencies [Line Items]
Plaintiff	Glowczenski
Month Served	Oct-04
Jurisdiction	US District Court, ED NY
Claim Type	Wrongful Death
Status	Notice of Appeal filed Sep. 2013; Opening Brief was filed Jan. 2014; Answering Brief filed Apr. 2014.
Mitchell [Member]
Loss Contingencies [Line Items]
Plaintiff	Mitchell
Month Served	Apr-12
Jurisdiction	US District Court, ED MI
Claim Type	Wrongful Death
Status	Notice of Appeal filed Aug. 2014