Stock Based Awards and Other Equity Instruments - Summary of Stock Option Activity (Details) - Stock Options $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Options Outstanding
Options Outstanding, Beginning balance | shares	6,853
Options Outstanding, Granted | shares	611
Options Outstanding, Exercised | shares	(196)	[1]
Options Outstanding, Cancelled or expired | shares	(72)
Options Outstanding, Ending balance | shares	7,196
Options Outstanding, Exercisable | shares	3,428
Options Outstanding, Vested and expected to vest | shares	7,196	[2]
Weighted Average Exercise Price per share
Options Outstanding, Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 52.78
Options Granted, Weighted Average Exercise Price | $ / shares	41.54
Options Exercised, Weighted Average Exercise Price | $ / shares	28.21	[1]
Options Cancelled or expired, Weighted Average Exercise Price | $ / shares	47.80
Options Outstanding, Weighted Average Exercise Price, Ending balance | $ / shares	52.48
Options Exercisable, Weighted Average Exercise Price | $ / shares	54.81
Options Vested and expected to vest, Weighted Average Exercise Price | $ / shares	$ 52.48	[2]
Weighted Average Remaining Contractual Life and Aggregate Intrinsic Value
Options Outstanding, Weighted Average Remaining Contractual Life	6 years 8 months 12 days
Options Exercisable, Weighted Average Remaining Contractual Life	4 years 8 months 12 days
Options Vested and expected to vest, Weighted Average Remaining Contractual Life	6 years 8 months 12 days	[2]
Options Outstanding, Aggregate Intrinsic Value | $	$ 10
Options Exercisable, Aggregate Intrinsic Value | $	5
Options Vested and expected to vest, Aggregate Intrinsic Value | $	$ 10	[2]

[1] Inclusive of 150,855 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 that had been convertible under stock options that were withheld under net share settlement remain in the authorized but unissued pool under the 2011 Incentive 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.