Changes in accumulated other comprehensive income (loss) - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Amounts reclassified from accumulated other comprehensive income (loss), pension and other postretirement plans, net of tax	$ (0.1)	$ (1.2)	$ (0.4)	$ (1.4)
Amounts reclassified from accumulated other comprehensive income, pension and other postretirement plans, tax	$ (0.1)	$ (0.7)	$ (0.3)	$ (0.9)