Share-Based Compensation - Narrative (Details) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	May 31, 2018 period shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Jul. 01, 2018 $ / shares	Feb. 28, 2018 shares
Class of Stock [Line Items]
Non-cash compensation expense | $		$ 130	$ 135	$ 124
Allocation of recognized period costs, capitalized amount | $		$ 15	18	13
Capital shares reserved for future issuance (in shares) | shares		36,600,000
Number of options, exercised (in shares) | shares		2,596,000
Weighted average exercise price (in dollars per share) | $ / shares		$ 40.22
Vested in period, fair value | $		$ 206	206	130
Employee stock option
Class of Stock [Line Items]
Award vesting period		3 years
Awards exercise period		10 years
Intrinsic value | $		$ 36	$ 22	$ 18
Compensation cost not yet recognized | $		$ 8
Compensation cost not yet recognized, Period for recognition		1 year 7 months 6 days
Number of options, exercised (in shares) | shares		2,600,000	3,000,000	2,800,000
Weighted average exercise price (in dollars per share) | $ / shares		$ 40.22	$ 36.36	$ 31.61
Employee stock option | Minimum
Class of Stock [Line Items]
Stock options granted, expiration period		14 days
Employee stock option | Maximum
Class of Stock [Line Items]
Stock options granted, expiration period		60 days
Employee stock
Class of Stock [Line Items]
Stock-based compensation | $		$ 4
Number of shares authorized (up to) (in shares) | shares	25,000,000
Percentage in increment of eligible pay	1.00%
Percentage of maximum contribution of eligible pay	25.00%
Maximum number of shares per employee (in shares) | shares	25,000
Shares employees are eligible for in each offering period (in shares) | shares	1,250
Number of offering periods per year | period	2
Percentage of purchase price of common stock	85.00%
Share price for current trading period (in dollars per share) | $ / shares		$ 75.33			$ 73.73
Discount from market price	15.00%
Time Based And Performance Based Restricted Stock
Class of Stock [Line Items]
Compensation cost not yet recognized | $		$ 116
Compensation cost not yet recognized, Period for recognition		1 year 2 months 12 days
Time Based Restricted Units
Class of Stock [Line Items]
Grants in period (in shares) | shares		1,153,069	2,364,288	2,325,985
Typical vesting period		3 years
Performance Based Restricted Stock Units
Class of Stock [Line Items]
Grants in period (in shares) | shares		700,000
Stock granted, value, net of forfeitures | $		$ 44
Typical vesting period		3 years
Stock-based compensation | $		$ 26
Performance Based Restricted Stock Units | Maximum | Restricted Stock
Class of Stock [Line Items]
Shares reserved for future issuance (in shares) | shares						1,300,000