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, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Beginning Outstanding, shares		1.1	1.3	1.1
Granted , shares		0.5	0.5	0.5
Performance Adjustment, shares	[1]	0.0	(0.1)	0.1
Lapsed, shares		(0.2)	(0.5)	(0.3)
Cancelled, shares		(0.2)	(0.1)	(0.1)
Ending Outstanding, shares		1.2	1.1	1.3
Performance Units Vested Not Yet Issued		0.1	0.0	0.3
Outstanding, Weighted Average Grant Date Fair Value		$ 38.24	$ 36.56	$ 31.70
Granted, Weighted Average Grant Date Fair Value		42.52	33.28	41.34
Performance Adjustment, Weighted Average Grant Date Fair Value		0	45.47	29.59
Lapsed, Weighted Average Grant Date Fair Value		41.42	29.86	24.09
Canceled, Weighted Average Grant Date Fair Value		41.75	39.20	35.89
Outstanding, Weighted Average Grant Date Fair Value		38.74	38.24	36.56
Weighted Average Grant Date Fair Value of Performance Units Vested Not Yet Issued		$ 42.90	$ 0	$ 29.59

[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 2015 that vested on December 31, 2017, PSUs granted in 2014 that vested on December 31, 2016 and PSUs granted in 2013 that vested on December 31, 2015.