Summary of Status of Senior Executive Plans (Detail) - $ / shares shares in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Shares
Granted		2.2	1.4	1.2
Vested		(1.2)	(0.9)	(0.8)
Restricted Stock | Executive Officer
Shares
Balance, at beginning of year		1.2	0.9	0.7
Granted		2.0	1.3	1.1
Vested	[1]	(1.1)	(0.7)	(0.7)
Forfeited/expired		(0.5)	(0.3)	(0.2)
Balance, at end of year		1.6	1.2	0.9
Issued in calendar year	[1]	0.4	0.4	0.3
Fair Value
Balance, at beginning of year		$ 13	$ 14	$ 14
Granted		10	14	16
Vested	[1]	15	19	17
Forfeited/expired		15	19	17
Balance, at end of year		10	13	14
Issued in calendar year	[1]	$ 19	$ 17	$ 20

[1] Shares that vest at December 31 of each year are issued to the employees in the first quarter of the following year, although the requisite service period is complete. Accordingly, the 0.4 million shares issued in 2020 include shares vested at December 31, 2019, after adjusting for shares withheld to meet employee tax requirements. The shares withheld for employee tax requirements were valued at $5.5 million, $5.4 million and $4.8 million for 2020, 2019 and 2018, respectively.