Stock Plans - Performance Share Units and Other Stock-Based Awards Activity (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended	9 Months Ended
		Sep. 30, 2019	Sep. 30, 2019
Number of Shares
Balance at January 1, 2019 (in shares)			6,559,010
Shares granted (in shares)			1,678,173
Vested (in shares)	[1]		(1,679,731)
Forfeited (in shares)	[1]		(633,821)
Balance at September 30, 2019 (in shares)		5,923,631	5,923,631
Weighted-average grant date fair value per share
Balance at January 1, 2019 (in dollars per share)	[2]		$ 42.19
Shares granted (in dollars per share)	[2]		53.65
Vested (in dollars per share)	[1],[2]		36.90
Forfeited (in dollars per share)	[1],[2]		45.86
Balance at September 30, 2019 (in dollars per share)	[2]	$ 46.54	$ 46.54
Weighted-Average Aggregate Fair Value
Weighted average grant date fair value of shares granted	[2]		$ 90
Weighted average grant date fair value of shares vested	[1],[2]		62
Maximum
Weighted-Average Aggregate Fair Value
Actual tax benefit/(expense) realized for the tax deductions from the shares vested		$ 1	$ 2
Annual grant to eligible employees
Grant Date
Grant Date			Feb. 22, 2019
Performance share units
Number of Shares
Shares granted (in shares)			891,210
Weighted-average grant date fair value per share
Shares granted (in dollars per share)	[2]		$ 57.91
Deferred stock units
Number of Shares
Shares granted (in shares)			666,880
Weighted-average grant date fair value per share
Shares granted (in dollars per share)	[2]		$ 47.72
Additional shares granted
Number of Shares
Shares granted (in shares)	[3]		120,083
Weighted-average grant date fair value per share
Shares granted (in dollars per share)	[2],[3]		$ 55.05

[1] Includes performance share units, deferred stock units and historically granted restricted stock. The actual tax benefit/(expense) realized and recorded in the provision for income taxes for the tax deductions from the shares vested totaled less than $1 million in the three months and $2 million in the nine months ended September 30, 2019.

[2] The grant date fair value of performance share units is determined based on the Monte Carlo simulation model for the market-based total shareholder return component and the closing market price of the Company’s stock on the grant date for performance-based components. The Monte Carlo simulation model incorporates the probability of achieving the total shareholder return market condition. Compensation expense is recognized using the grant date fair values regardless of whether the market condition is achieved, so long as the requisite service has been provided.

[3] Includes performance share units and deferred stock units.