Commitments and contingencies - Summary of Cases Dismissed or Judgment Entered (Detail)	6 Months Ended
	Jun. 30, 2014
Turner [Member]
Loss Contingencies [Line Items]
Plaintiff	Turner
Month Served	Feb-10
Jurisdiction	US District Court, ED NC
Claim Type	Wrongful Death
Status	Settled and dismissed
Salgado [Member]
Loss Contingencies [Line Items]
Plaintiff	Saldago
Month Served	Apr-14
Jurisdiction	Harris County District Court, TX
Claim Type	Wrongful Death
Status	Voluntary Dismissal
Faltesek [Member]
Loss Contingencies [Line Items]
Plaintiff	Faltesek
Month Served	Apr-14
Jurisdiction	Harris County District Court, TX PA
Claim Type	Wrongful Death
Status	Voluntary Dismissal
Mitchell [Member]
Loss Contingencies [Line Items]
Plaintiff	Mitchell
Month Served	Apr-12
Jurisdiction	US District Court, ED MI
Claim Type	Wrongful Death
Status	Motion for Summary Judgement Granted
McCue [Member]
Loss Contingencies [Line Items]
Plaintiff	McCue
Month Served	Mar-14
Jurisdiction	US District Court, District of Maine
Claim Type	Wrongful Death
Status	Voluntary Dismissal