Long-Term Incentive Employee Compensation Summary of Stock Options by Exercise Price (Detail) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options Outstanding Number	0.2	0.3	0.7	0.9
Options Outstanding Weighted-Average Remaining Contractual Life (In Years)	3 years 2 months 12 days
Options Outstanding Aggregate Intrinsic Value	$ 7.4
Options Exercisable Number	0.2	0.3	0.5
Options Exercisable Weighted-Average Remaining Contractual Life (In Years)	3 years 3 months 18 days
Options Exercisable Aggregate Intrinsic Value	$ 7.4
$19.97 - $26.76
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options Outstanding Number	0.1
Options Outstanding Weighted-Average Remaining Contractual Life (In Years)	1 year 7 months 6 days
Options Outstanding Aggregate Intrinsic Value	$ 3.7
Options Exercisable Number	0.1
Options Exercisable Weighted-Average Remaining Contractual Life (In Years)	1 year 7 months 6 days
Options Exercisable Aggregate Intrinsic Value	$ 3.7
$33.01 - $43.52
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options Outstanding Number	0.1
Options Outstanding Weighted-Average Remaining Contractual Life (In Years)	4 years 4 months 24 days
Options Outstanding Aggregate Intrinsic Value	$ 3.7
Options Exercisable Number	0.1
Options Exercisable Weighted-Average Remaining Contractual Life (In Years)	4 years 4 months 24 days
Options Exercisable Aggregate Intrinsic Value	$ 3.7