Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Discount rate from fair value on purchase date that participants pay for shares.

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Number of shares authorized for issuance under share-based payment arrangement.

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Stock-Based Compensation - Additional Information (Detail) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense	$ 147
Weighted average period to recognize expense	2 years 4 months 24 days
Equity Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)	4 years
Unrecognized compensation expense	$ 6
Weighted average period to recognize expense	2 years 2 months 12 days
Performance Based Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)	3 years
Unrecognized compensation expense	$ 24
Weighted average period to recognize expense	1 year 9 months 18 days
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserve for issuance or grant (shares)	6.0
Discount allowed to employees on purchase of shares under ESPP plan	5.00%	5.00%	5.00%
1998 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserve for issuance or grant (shares)	33.0
2001 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserve for issuance or grant (shares)	50.6
2001 Plan \| Instruments Other Than Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserve for issuance or grant (shares)	14.0
Directors' Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserve for issuance or grant (shares)	1.7
Award vesting period (in years)	1 year
Minimum \| Equity Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)	1 year
Maximum \| Equity Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)	4 years
Maximum \| 1998 and 2001 Plan \| Equity Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)	10 years