Stock Based Awards and Other Equity Instruments - Summary of Stock Option Activity (Details) - Stock Options - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Options Outstanding
Options Outstanding, Beginning balance		6,017	6,041	6,853
Options Outstanding, Granted		1,106	752	762
Options Outstanding, Exercised	[1]	(4)	(195)	(1,162)
Options Outstanding, Cancelled or expired		(1,504)	(581)	(412)
Options Outstanding, Ending balance		5,615	6,017	6,041
Options Outstanding, Exercisable		3,293
Options Outstanding, Vested and expected to vest	[2]	5,615
Weighted Average Exercise Price per share
Options Outstanding, Weighted Average Exercise Price, Beginning Balance		$ 50.27	$ 54.00	$ 52.78
Options Granted, Weighted Average Exercise Price		25.23	48.30	43.53
Options Exercised, Weighted Average Exercise Price	[1]	22.94	42.17	37.26
Options Cancelled or expired, Weighted Average Exercise Price		46.72	56.97	61.46
Options Outstanding, Weighted Average Exercise Price, Ending balance		46.31	$ 50.27	$ 54.00
Options Exercisable, Weighted Average Exercise Price		55.87
Options Vested and expected to vest, Weighted Average Exercise Price	[2]	$ 46.31
Weighted Average Remaining Contractual Life and Aggregate Intrinsic Value
Options Outstanding, Weighted Average Remaining Contractual Life		5 years 3 months 18 days
Options Exercisable, Weighted Average Remaining Contractual Life		3 years 4 months 24 days
Options Vested and expected to vest, Weighted Average Remaining Contractual Life	[2]	5 years 3 months 18 days
Options Outstanding, Aggregate Intrinsic Value		$ 3
Options Vested and expected to vest, Aggregate Intrinsic Value	[2]	$ 3

[1] Inclusive of 2,217, 120,112, and 814,635 stock options as of December 31, 2020, 2019 and 2018, respectively, 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 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.