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, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Outstanding – January 1		1.2	1.2	1.1
Granted		0.3	0.4	0.5
Performance adjustment(a)	[1]	0.1	0.0	0.0
Vested and issued		(0.6)	(0.3)	(0.2)
Forfeited		0.0	(0.1)	(0.2)
Outstanding – December 31		1.0	1.2	1.2
Vested pending issuance		0.2	0.2	0.1
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		$ 42.94	$ 38.74	$ 38.24
Granted		60.91	54.79	42.52
Performance adjustment(a)		44.87	0	0
Vested and issued		38.03	41.09	41.42
Forfeited		0	42.55	41.75
Outstanding – December 31		51.24	42.94	38.74
Vested pending issuance		$ 44.87	$ 33.27	$ 42.90

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