Pension and Postretirement Benefit Plans - Summary of Pension and Postretirement Benefit Plans' Change in Benefit Obligation, Plan Assets and Funded Status (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Change in benefit obligation:
Actuarial (gain) loss	$ 5.3	$ (54.4)	$ 32.5
Change in plan assets available for plan benefits:
Employer contribution	13.0
Pension Costs
Change in benefit obligation:
Benefit obligation at beginning of year	292.5	230.5
Service cost	7.7	4.6
Interest cost	11.1	10.8
Actuarial (gain) loss	(17.6)	57.3
Benefits paid	(12.4)	(10.7)
The Company’s portion of benefit obligation at end of year	281.3	292.5	230.5
Change in plan assets available for plan benefits:
Fair value of plan assets available for plan benefits at beginning of year	205.1	185.9
Employer contribution	13.0	13.0
Actual return on plan assets	(1.3)	16.9
Benefits paid	(12.4)	(10.7)
The Company’s portion of fair value of plan assets at end of year	204.4	205.1	185.9
Supplemental executive retirement plan	(6.8)	(6.3)
Funded status at end of year	(83.7)	(93.7)
Accumulated benefit obligation end of year	263.1	260.1
Postretirement Benefit Costs
Change in benefit obligation:
Benefit obligation at beginning of year	23.9	22.3
Service cost	0.0	0.0
Interest cost	0.9	1.1
Actuarial (gain) loss	(3.3)	1.7
Benefits paid	(1.2)	(1.2)
The Company’s portion of benefit obligation at end of year	20.3	23.9	22.3
Change in plan assets available for plan benefits:
Fair value of plan assets available for plan benefits at beginning of year	0.3	1.3
Employer contribution	0.0	0.0
Actual return on plan assets	0.0	0.0
Benefits paid	(0.3)	(1.0)
The Company’s portion of fair value of plan assets at end of year	0.0	0.3	$ 1.3
Supplemental executive retirement plan	0.0	0.0
Funded status at end of year	$ (20.3)	$ (23.6)