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, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Outstanding – January 1		1.2	1.1	1.3
Granted		0.4	0.5	0.5
Performance adjustment(a)	[1]	0.0	0.0	(0.1)
Vested and issued		(0.3)	(0.2)	(0.5)
Forfeited		(0.1)	(0.2)	(0.1)
Outstanding – December 31		1.2	1.2	1.1
Vested pending issuance		0.2	0.1	0.0
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		$ 38.74	$ 38.24	$ 36.56
Granted		54.79	42.52	33.28
Performance adjustment(a)		0	0	45.47
Vested and issued		41.09	41.42	29.86
Forfeited		42.55	41.75	39.20
Outstanding – December 31		42.94	38.74	38.24
Vested pending issuance		$ 33.27	$ 42.90	$ 0

[1] (a)Represents the adjustment to the number of shares to be issued above or below target for performance results achieved relative to PSUs granted in 2016 that vested on December 31, 2018, PSUs, granted in 2015 that vested on December 31, 2017, and PSUs granted in 2014 that vested on December 31, 2016.