Stock-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock-based Compensation Arrangements Activity
Stock-based compensation expense	$ 80	$ 89	$ 86
Tax benefit recognized on stock-based compensation expense	13	19	18
Tax benefit realized for tax deductions resulting from exercises and vestings	1	17	32
Restricted Stock [Member]
Stock-based Compensation Arrangements Activity
Stock-based compensation expense	$ 63	$ 64	$ 63
Number of Shares
Nonvested shares as of beginning of period (shares)	1,091,854
Granted (shares)	1,126,483
Vested (shares)	(770,727)
Forfeited (shares)	(9,698)
Nonvested shares as of end of period (shares)	1,437,912	1,091,854
Weighted- Average Grant-Date Fair Value Per Share
Nonvested shares as of beginning of period (in usd per share)	$ 93.38
Granted (in usd per share)	55.62	$ 98.75	$ 92.12
Vested (in usd per share)	82.80
Forfeited (in usd per share)	93.73
Nonvested shares as of end of period (in usd per share)	$ 69.47	$ 93.38
Vested Awards Other Than Options Rollforward
Fair value of restricted stock vested (in millions)	$ 35	$ 74	$ 80
Stock Based Compensation (Textual)
Vesting period of stock-based payment awards granted	3 years
Unrecognized share-based compensation cost related to outstanding unvested awards	$ 57
Weighted-average period of recognition for unrecognized compensation costs on nonvested awards	2 years
Performance Awards [Member]
Stock-based Compensation Arrangements Activity
Stock-based compensation expense	$ 15	23	22
Stock Options and Other Awards [Member]
Stock-based Compensation Arrangements Activity
Stock-based compensation expense	$ 2	$ 2	$ 1
Omnibus Stock Incentive Plan [Member] \| Share-based Payment Arrangement [Member]
Stock Based Compensation (Textual)
Number of shares of common stock available to be awarded under stock-based compensation plans (shares)	14,787,213

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

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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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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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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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