Income taxes - Open Tax Years - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2013
Period Start [Member] | Internal Revenue Service [Member]
Income Tax Examination [Line Items]
Open tax year	2009
Period Start [Member] | State tax jurisdictions [Member]
Income Tax Examination [Line Items]
Open tax year	2008
Period Start [Member] | Netherlands tax authority [Member]
Income Tax Examination [Line Items]
Open tax year	2010
Period End [Member] | Internal Revenue Service [Member]
Income Tax Examination [Line Items]
Open tax year	2012
Period End [Member] | State tax jurisdictions [Member]
Income Tax Examination [Line Items]
Open tax year	2012
Period End [Member] | Netherlands tax authority [Member]
Income Tax Examination [Line Items]
Open tax year	2012