Income taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014
Income Tax Disclosure [Line Items]
Total current and long term deferred tax assets	$ 16.8
Research and development tax credit studies	10.4
Liability for unrecognized tax benefits	3.3
Overall effective tax rate, after discrete period adjustments	36.90%
Effective tax rate, before discrete period adjustments	37.60%
Federal [Member]
Income Tax Disclosure [Line Items]
Liability for unrecognized tax benefits	3.2
State [Member]
Income Tax Disclosure [Line Items]
Liability for unrecognized tax benefits	0.1
Research and Development Credits [Member]
Income Tax Disclosure [Line Items]
Unrecognized tax benefit associated with research and development credits that has been netted against the research and development credit deferred tax asset	$ 1.9