Commitments and contingencies - Summary of Cases Dismissed or Judgment Entered (Detail)	9 Months Ended
	Sep. 30, 2014
Faltesek [Member]
Loss Contingencies [Line Items]
Plaintiff	Faltesek
Month Served	Apr-14
Jurisdiction	Harris County District Court, TX
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 Judgment 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
Slade [Member]
Loss Contingencies [Line Items]
Plaintiff	Slade
Month Served	Dec-13
Jurisdiction	US District Court, ED TX
Claim Type	Wrongful Death
Status	Voluntary Dismissal
Goodard [Member]
Loss Contingencies [Line Items]
Plaintiff	Goodard
Month Served	Jul-14
Jurisdiction	Pinellas County Circuit Court, FL
Claim Type	Wrongful Death
Status	Voluntary Dismissal
Parker [Member]
Loss Contingencies [Line Items]
Plaintiff	Parker
Month Served	Jul-14
Jurisdiction	US District Court, SD TX
Claim Type	Suspect Injury During Arrest
Status	Voluntary Dismissal