Long-Term Incentive Employee Compensation Rollforward of Outstanding Stock Options (Detail) - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock Options Outstanding Rollforward - Shares
Outstanding – January 1	0.3	0.7	0.9
Exercised	(0.1)	(0.4)	(0.2)
Outstanding – December 31	0.2	0.3	0.7
Options exercisable – December 31	0.2	0.3	0.5
Stock Options Outstanding Rollforward - Weighted Average Exercise Price
Outstanding – January 1	$ 33.55	$ 35.04	$ 34.07
Exercised	35.37	36.08	30.52
Outstanding – December 31	32.22	33.55	35.04
Options exercisable – December 31	$ 32.22	$ 33.55	$ 36.04