Commitments and contingencies - Information Regarding the Company's Insurance Coverage (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014
2004 [Member]
Product Liability Contingency [Line Items]
Policy Year	2004
Policy Start Date	Dec. 01, 2003
Policy End Date	Dec. 01, 2004
Insurance Coverage	$ 2.0
Deductible Amount	0.1
Defense Costs Covered	N
Remaining Insurance Coverage	2.0
Active Cases and Cases on Appeal	Glowczenski
2005 [Member]
Product Liability Contingency [Line Items]
Policy Year	2005
Policy Start Date	Dec. 01, 2004
Policy End Date	Dec. 01, 2005
Insurance Coverage	10.0
Deductible Amount	0.3
Defense Costs Covered	Y
Remaining Insurance Coverage	7.0
Active Cases and Cases on Appeal	n/a
2006 [Member]
Product Liability Contingency [Line Items]
Policy Year	2006
Policy Start Date	Dec. 01, 2005
Policy End Date	Dec. 01, 2006
Insurance Coverage	10.0
Deductible Amount	0.3
Defense Costs Covered	Y
Remaining Insurance Coverage	3.7
Active Cases and Cases on Appeal	n/a
2007 [Member]
Product Liability Contingency [Line Items]
Policy Year	2007
Policy Start Date	Dec. 01, 2006
Policy End Date	Dec. 01, 2007
Insurance Coverage	10.0
Deductible Amount	0.3
Defense Costs Covered	Y
Remaining Insurance Coverage	8.0
Active Cases and Cases on Appeal	n/a
2008 [Member]
Product Liability Contingency [Line Items]
Policy Year	2008
Policy Start Date	Dec. 01, 2007
Policy End Date	Dec. 15, 2008
Insurance Coverage	10.0
Deductible Amount	0.5
Defense Costs Covered	Y
Remaining Insurance Coverage
Active Cases and Cases on Appeal	Grable, Koon
2009 [Member]
Product Liability Contingency [Line Items]
Policy Year	2009
Policy Start Date	Dec. 01, 2008
Policy End Date	Dec. 15, 2009
Insurance Coverage	10.0
Deductible Amount	1.0
Defense Costs Covered	N
Remaining Insurance Coverage	10.0
Active Cases and Cases on Appeal	Derbyshire
2010 [Member]
Product Liability Contingency [Line Items]
Policy Year	2010
Policy Start Date	Dec. 15, 2009
Policy End Date	Dec. 15, 2010
Insurance Coverage	10.0
Deductible Amount	1.0
Defense Costs Covered	N
Remaining Insurance Coverage	10.0
Active Cases and Cases on Appeal	Thompson, Shymko, Doan, Juran
2011 [Member]
Product Liability Contingency [Line Items]
Policy Year	2011
Policy Start Date	Dec. 15, 2010
Policy End Date	Dec. 15, 2011
Insurance Coverage	10.0
Deductible Amount	1.0
Defense Costs Covered	N
Remaining Insurance Coverage	10.0
Active Cases and Cases on Appeal	Wilson
Jan - Jun 2012 [Member]
Product Liability Contingency [Line Items]
Policy Start Date	Dec. 15, 2011
Policy End Date	Jun. 25, 2012
Insurance Coverage	7.0
Deductible Amount	1.0
Defense Costs Covered	N
Remaining Insurance Coverage	7.0
Active Cases and Cases on Appeal	Ramsey, Firman, Ricks, Mitchell
Jul - Dec 2012 [Member]
Product Liability Contingency [Line Items]
Policy Start Date	Jun. 25, 2012
Policy End Date	Dec. 15, 2012
Insurance Coverage	12.0
Deductible Amount	1.0
Defense Costs Covered	N
Remaining Insurance Coverage	12.0
Active Cases and Cases on Appeal	n/a
2013 [Member]
Product Liability Contingency [Line Items]
Policy Year	2013
Policy Start Date	Dec. 15, 2012
Policy End Date	Dec. 15, 2013
Insurance Coverage	12.0
Deductible Amount	1.0
Defense Costs Covered	N
Remaining Insurance Coverage	12.0
Active Cases and Cases on Appeal	Miller
2014 [Member]
Product Liability Contingency [Line Items]
Policy Year	2014
Policy Start Date	Dec. 15, 2013
Policy End Date	Dec. 15, 2014
Insurance Coverage	12.0
Deductible Amount	4.0
Defense Costs Covered	N
Remaining Insurance Coverage	$ 12.0
Active Cases and Cases on Appeal	Rascom, Schrock, Moore, Ward
Minimum [Member] | Jan - Jun 2012 [Member]
Product Liability Contingency [Line Items]
Policy Month Year	2012-01
Minimum [Member] | Jul - Dec 2012 [Member]
Product Liability Contingency [Line Items]
Policy Month Year	2012-07
Maximum [Member] | Jan - Jun 2012 [Member]
Product Liability Contingency [Line Items]
Policy Month Year	2012-06
Maximum [Member] | Jul - Dec 2012 [Member]
Product Liability Contingency [Line Items]
Policy Month Year	2012-12