Long-Term Incentive Employee Compensation Rollforward of Outstanding Restricted Stock (Detail) - RSUs, PSUs and RSAs [Member] - $ / shares shares in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Outstanding – January 1		1.0	1.2	1.2
Granted		0.3	0.3	0.4
Performance adjustment(a)	[1]	0.1	0.1	0.0
Vested and issued		(0.5)	(0.6)	(0.3)
Forfeited		(0.1)	0.0	(0.1)
Outstanding – December 31		0.8	1.0	1.2
Vested pending issuance		0.2	0.2	0.2
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Outstanding – January 1		$ 51.24	$ 42.94	$ 38.74
Granted		61.13	60.91	54.79
Performance adjustment(a)		57.88	44.87	0
Vested and issued		44.86	38.03	41.09
Forfeited		59.50	0	42.55
Outstanding – December 31		59.25	51.24	42.94
Vested pending issuance		$ 57.88	$ 44.87	$ 33.27

[1] Represents an adjustment for performance results achieved related to outstanding PSU shares that vested during the period and are pending issuance.