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Employee Stock-based Compensation Plans - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted in period (in shares)	0	0	0
2012 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized to be issued (in shares)	27,500,000
New Employee Stock Inducement Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized to be issued (in shares)	1,300,000
Employee Stock Purchase Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee stock purchase program discount from average market price (as a percentage)	15.00%
Percentage of authorized payroll deductions for common stock purchases by employees (as a percentage)	10.00%
Shares authorized to be issued under the Employee Stock Purchase Program (in shares)	7,000,000
Remaining shares authorized to be issued under the Employee Stock Purchase Program (in shares)	1,300,000
Service-Based Awards | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	3 years
Performance Based Awards | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	1 year
Performance Based Awards | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	3 years
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost related to unvested stock grants	$ 122
Cost expected to be recognized over a weighted-average period (in years)	1 year

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

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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

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