Changes in accumulated other comprehensive income (loss) - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Amounts reclassified from accumulated other comprehensive income, pension and other postretirement plans, net of tax	$ (0.1)	$ 0.1	$ (0.3)	$ (0.2)
Amounts reclassified from accumulated other comprehensive income, pension and other postretirement plans, tax	$ 0.1	$ 0.1	$ (0.2)	$ (0.1)