Stock Based Awards and Other Equity Instruments - Summary of Stock Option Activity (Details) - Stock Options - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		9 Months Ended
		Sep. 30, 2019
Options Outstanding
Options Outstanding, Beginning balance		6,041
Options Outstanding, Granted		687
Options Outstanding, Exercised	[1]	(193)
Options Outstanding, Cancelled or expired		(503)
Options Outstanding, Ending balance		6,032
Options Outstanding, Exercisable		3,361
Options Outstanding, Vested and expected to vest	[2]	6,032
Weighted Average Exercise Price per share
Options Outstanding, Weighted Average Exercise Price, Beginning Balance		$ 54.00
Options Granted, Weighted Average Exercise Price		53.09
Options Exercised, Weighted Average Exercise Price	[1]	42.19
Options Cancelled or expired, Weighted Average Exercise Price		57.49
Options Outstanding, Weighted Average Exercise Price, Ending balance		53.98
Options Exercisable, Weighted Average Exercise Price		60.35
Options Vested and expected to vest, Weighted Average Exercise Price	[2]	$ 53.98
Weighted Average Remaining Contractual Life and Aggregate Intrinsic Value
Options Outstanding, Weighted Average Remaining Contractual Life		6 years 1 month 6 days
Options Exercisable, Weighted Average Remaining Contractual Life		4 years 3 months 18 days
Options Vested and expected to vest, Weighted Average Remaining Contractual Life	[2]	6 years 1 month 6 days
Options Outstanding, Aggregate Intrinsic Value		$ 3
Options Vested and expected to vest, Aggregate Intrinsic Value	[2]	$ 3

[1] Inclusive of 120,000 of options which were not converted into shares due to net share settlement in order to cover the aggregate exercise price and the required amount of employee withholding taxes. Potential shares which had been convertible under stock options that were withheld under net share settlement remain in the authorized but unissued pool under the 2018 Plan and can be reissued by the Company. Total payments for the employees’ tax obligations to the taxing authorities due to net share settlements are reflected as a financing activity within the unaudited condensed consolidated statements of cash flows.

[2] The Company accounts for forfeitures as they occur, rather than estimate expected forfeitures as allowed under GAAP and   therefore do not include a forfeiture rate in our vested and expected to vest calculation unless necessary for a performance condition award.