Long-Term Incentive Employee Compensation Summary of Stock Options by Exercise Price (Detail) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding Number	0.9	1.4	1.7	1.9
Options Outstanding Weighted-Average Remaining Contractual Life (In Years)	6 years 6 months
Options Outstanding Aggregate Intrinsic Value	$ 17.3
Options Exercisable Number	0.5	0.8	1.1
Options Exercisable Weighted-Average Remaining Contractual Life (In Years)	5 years 4 months 24 days
Options Exercisable Aggregate Intrinsic Value	$ 10.0
Less than $23.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding Number	0.1
Options Outstanding Weighted-Average Remaining Contractual Life (In Years)	3 years 6 months
Options Outstanding Aggregate Intrinsic Value	$ 3.4
Options Exercisable Number	0.1
Options Exercisable Weighted-Average Remaining Contractual Life (In Years)	3 years 6 months
Options Exercisable Aggregate Intrinsic Value	$ 3.4
$26.76 Exercise Price
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding Number	0.1
Options Outstanding Weighted-Average Remaining Contractual Life (In Years)	5 years 2 months 12 days
Options Outstanding Aggregate Intrinsic Value	$ 4.2
Options Exercisable Number	0.1
Options Exercisable Weighted-Average Remaining Contractual Life (In Years)	5 years 2 months 12 days
Options Exercisable Aggregate Intrinsic Value	$ 4.2
$33.01 Exercise Price
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding Number	0.3
Options Outstanding Weighted-Average Remaining Contractual Life (In Years)	8 years 1 month 6 days
Options Outstanding Aggregate Intrinsic Value	$ 6.1
Options Exercisable Number	0.1
Options Exercisable Aggregate Intrinsic Value	$ 0.6
$41.52 Exercise Price
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding Number	0.2
Options Outstanding Weighted-Average Remaining Contractual Life (In Years)	7 years 2 months 12 days
Options Outstanding Aggregate Intrinsic Value	$ 2.2
Options Exercisable Number	0.1
Options Exercisable Aggregate Intrinsic Value	$ 0.4
$43.52 Exercise Price
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding Number	0.2
Options Outstanding Weighted-Average Remaining Contractual Life (In Years)	6 years 2 months 12 days
Options Outstanding Aggregate Intrinsic Value	$ 1.4
Options Exercisable Number	0.1
Options Exercisable Aggregate Intrinsic Value	$ 1.4