Commitments and contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Cases
Other Environmental Cleanup Matters [Member]
Commitments And Contingent Liabilities [Line Items]
Accrual for other environmental cleanup matters	2.1
NL [Member]
Commitments And Contingent Liabilities [Line Items]
Number of cases settled and dismissed and found not liable	100
Period by which loss contingency claims settled and dismissed	20 years
NL [Member] | Pending Claims [Member] | Product Liability and Occupational Exposure Litigation Claims [Member]
Commitments And Contingent Liabilities [Line Items]
Type of cases pending	1,125
Number of plaintiffs involved	1,643
NL [Member] | Administratively Dismissed Claims [Member] | Product Liability and Occupational Exposure Litigation Claims [Member]
Commitments And Contingent Liabilities [Line Items]
Number of plaintiffs involved	8,298
NL [Member] | Environmental Remediation Sites Nl Named As Prp Or Defendant [Member]
Commitments And Contingent Liabilities [Line Items]
Accrual for other environmental cleanup matters	48.0
Number of sites associated with remediation ad related costs	50
Upper end range, estimate costs for remediation and related matters	139
Number of sites for which NL not currently able to estimate costs	5