Stock-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-based Compensation Arrangements Activity
Stock-based compensation expense	$ 86	$ 77	$ 68
Tax benefit recognized on stock-based compensation expense	18	27	24
Tax benefit realized for tax deductions resulting from exercises and vestings	32	44	33
Effect of tax deductions in excess of recognized stock-based compensation expense	20	24	22
Restricted Stock [Member]
Stock-based Compensation Arrangements Activity
Stock-based compensation expense	$ 63	$ 58	$ 52
Number of Shares
Beginning balance (shares)	1,401,040
Granted (shares)	628,908
Vested (shares)	(843,709)
Forfeited (shares)	(9,661)
Ending balance (shares)	1,176,578	1,401,040
Weighted- Average Grant-Date Fair Value Per Share
Beginning balance (in usd per share)	$ 69.82
Granted (in usd per share)	92.12	$ 79.32	$ 59.00
Vested (in usd per share)	71.26
Forfeited (in usd per share)	69.97
Ending balance (in usd per share)	$ 80.70	$ 69.82
Vested Awards Other Than Options Rollforward
Fair value of restricted stock vested (in millions)	$ 80	$ 71	$ 46
Stock Based Compensation (Textual)
Vesting period of stock-based payment awards granted	3 years
Unrecognized share-based compensation cost related to outstanding unvested awards	$ 56
Weighted-average period of recognition for unrecognized compensation costs on nonvested awards	2 years
Restricted Stock Awards - Nonemployee [Member]
Stock Based Compensation (Textual)
Vesting period of stock-based payment awards granted	3 years
Performance Awards [Member]
Stock-based Compensation Arrangements Activity
Stock-based compensation expense	$ 22	19	15
Stock Options and Other Awards [Member]
Stock-based Compensation Arrangements Activity
Stock-based compensation expense	$ 1	$ 0	$ 1
Omnibus Stock Incentive Plan [Member] | Stock Compensation Plan [Member]
Stock Based Compensation (Textual)
Number of shares of common stock available to be awarded under stock-based compensation plans (shares)	8,532,542
Formerly Maintained Plans [Member] | Stock Compensation Plan [Member]
Stock Based Compensation (Textual)
Number of shares of common stock available to be awarded under stock-based compensation plans (shares)	0