Stock-Based Compensation - Restricted Stock Units (RSUs (Details) - RSUs - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock-Based Compensation
Award vesting period	3 years
Units
Non-vested at beginning of period (in shares)	717,239
Granted (in shares)	271,718
Forfeited (in shares)	(16,750)
Vested (in shares)	(452,693)
Non-vested at end of period (in shares)	519,514	717,239
Weighted Average Grant Date Fair Value per Unit
Non-vested at beginning of period (in usd per share)	$ 25.56
Granted (in usd per share)	38.05	$ 37.37	$ 28.90
Forfeited (in usd per share)	35.86
Vested (in usd per share)	20.73
Non-vested at end of period (in usd per share)	$ 35.87	$ 25.56
Vested in period, fair value	$ 17	$ 36	$ 42
Weighted average grant date fair value (in usd per share)	$ 38.05	$ 37.37	$ 28.90

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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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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