Income taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2014
Income Tax Disclosure [Line Items]
Total current and long term deferred tax assets	$ 18.4
Research and development tax credit studies	10.1
Liability for unrecognized tax benefits	3.2
Overall effective tax rate, after discrete period adjustments	34.70%
Effective tax rate, before discrete period adjustments	38.90%
Federal [Member]
Income Tax Disclosure [Line Items]
Liability for unrecognized tax benefits	3.1
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	$ 1.3