Stock Plans - Performance Share Units and Other Stock-Based Awards Activity (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Number of Shares
Balance at January 1, 20198 (in shares) | shares	6,559,010
Shares granted (in shares) | shares	1,576,849
Vested (in shares) | shares	(1,557,059)	[1]
Forfeited (in shares) | shares	(226,017)	[1]
Balance at March 31, 2019 (in shares) | shares	6,352,783
Weighted-average grant date fair value per share
Balance at January 1, 2019 (in dollars per share) | $ / shares	$ 42.19	[2]
Shares granted (in dollars per share) | $ / shares	53.40	[2]
Vested (in dollars per share) | $ / shares	36.42	[1],[2]
Forfeited (in dollars per share) | $ / shares	43.75	[1],[2]
Balance at March 31, 2019 (in dollars per share) | $ / shares	$ 46.33	[2]
Weighted-Average Aggregate Fair Value
Weighted average grant date fair value of shares granted | $	$ 84	[2]
Weighted average grant date fair value of shares vested | $	57	[1],[2]
Maximum
Weighted-Average Aggregate Fair Value
Actual tax benefit/(expense) realized for the tax deductions from the shares vested | $	$ 2
Annual grant to eligible employees
Grant Date
Grant Date	Feb. 22, 2019
Performance share units
Number of Shares
Shares granted (in shares) | shares	891,210
Weighted-average grant date fair value per share
Shares granted (in dollars per share) | $ / shares	$ 57.91	[2]
Deferred stock units
Number of Shares
Shares granted (in shares) | shares	666,880
Weighted-average grant date fair value per share
Shares granted (in dollars per share) | $ / shares	$ 47.72	[2]
Additional shares granted
Number of Shares
Shares granted (in shares) | shares	18,759	[3]
Weighted-average grant date fair value per share
Shares granted (in dollars per share) | $ / shares	$ 41.68	[2],[3]

[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 $2 million in the three months ended March 31, 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.