Postretirement Benefits Other Than Pensions - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2001
Defined Benefit Plan Disclosure [Line Items]
Minimum age of retirement under option one to avail benefit of post retirement	55 years
Minimum service period served at the time of retirement under option one to avail benefit of post retirement	15 years
Minimum age of retirement under option two to avail benefit of post retirement	65 years
Minimum service period served at time of retirement under option two to avail benefit of post retirement	10 years
Minimum for age plus years of service of retirement under amended plan to avail benefit of post retirement				60 years
Minimum service period served at the time of retirement under amended plan to avail benefit of post retirement				10 years
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected contribution to plan in 2014	$ 0.4
Net gain(loss) recognized in other comprehensive income	2.8	$ (0.9)	$ (0.3)
Amortization of net loss	0.3	0.4	0.5
Amortization of prior service credit recognized in other comprehensive income	1.3	1.3	$ 1.3
Unrecognized actuarial losses	3.7	6.8
Unrecognized actuarial losses, net of tax	2.3	4.1
Unrecognized Prior service credit	15.1	16.4
Unrecognized Prior service credit, Net of Tax	$ 9.2	$ 9.9