Defined Benefit Plans - Components of Net Periodic Benefit Cost for Pension and Postretirement Benefits (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	12 Months Ended
	Sep. 30, 2015	Mar. 31, 2014	Jun. 30, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Recognition of actuarial losses (gains)	$ 2.5			$ 8.6	$ 13.7	$ 5.2
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost				11.5	10.4
Interest cost				33.7	32.9
Recognition of actuarial losses (gains)				(54.1)	133.1
Amortization of prior service cost (credits)				0.0
Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost				0.1	0.1
Interest cost				0.6	0.8
Recognition of actuarial losses (gains)		$ 0.6	$ 4.0	(1.3)	3.9
Amortization of prior service cost (credits)				(5.5)
Net Periodic Benefit Cost | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost				11.5	10.4	11.4
Interest cost				33.7	32.9	30.1
Expected return on plan assets				(40.2)	(42.2)	(41.8)
Recognition of actuarial losses (gains)				2.9	12.5	0.8
Amortization of prior service cost (credits)				0.1	0.1	0.1
Curtailment and settlement losses						0.1
Net periodic benefit cost				8.0	13.7	0.7
Net Periodic Benefit Cost | Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost				0.1	0.1	0.3
Interest cost				0.6	0.8	1.7
Recognition of actuarial losses (gains)				(0.4)	1.2	4.4
Amortization of prior service cost (credits)				(13.5)	(27.6)	(27.4)
Curtailment and settlement losses						0.1
Net periodic benefit cost				$ (13.2)	$ (25.5)	$ (20.9)