Stock Plans - Deferred Stock Units, Performance Share Units and Restricted Stock (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Number of Shares
Beginning balance (in shares)		6,559,010	7,669,705		7,593,627
Shares granted (in shares)		1,763,163	2,283,832		3,470,323
Vested (in shares)	[1]	(2,007,848)	(2,511,992)	[2]	(2,622,807)	[2]
Forfeited (in shares)		(652,380)	(882,535)	[2]	(771,438)	[2]
Ending balance (in shares)		5,661,945	6,559,010		7,669,705
Weighted-Average Fair Value Per Share
Beginning balance (in dollars per share)	[3]	$ 42.19	$ 39.74		$ 36.90
Shares granted (in dollars per share)	[3]	53.69	46.72		42.75
Vested (in dollars per share)	[3]	37.81	38.91		35.78
Forfeited (in dollars per share)	[3]	45.88	42.00		38.69
Ending balance (in dollars per share)	[3]	$ 46.90	$ 42.19		$ 39.74
Weighted-Average Aggregate Fair Value
Total shares granted		$ 95	$ 107		$ 148
Vested		76	98		94
Actual tax benefit realized for the tax deductions from the shares vested		$ 2	$ 3		$ 7
Performance share units
Number of Shares
Shares granted (in shares)		891,210	1,048,770		1,087,010
Weighted-Average Fair Value Per Share
Shares granted (in dollars per share)	[3]	$ 57.91	$ 51.23		$ 43.14
Deferred stock units
Number of Shares
Shares granted (in shares)		666,880	788,310		845,550
Weighted-Average Fair Value Per Share
Shares granted (in dollars per share)	[3]	$ 47.72	$ 43.51		$ 43.20
Additional shares granted
Number of Shares
Shares granted (in shares)	[4]	205,073	446,752		1,537,763
Weighted-Average Fair Value Per Share
Shares granted (in dollars per share)	[3]	$ 54.81	$ 41.78		$ 42.22
Annual grant to eligible employees
Grant Date
Grant date		Feb. 22, 2019	Feb. 22, 2018		Feb. 16, 2017

[1] 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 2019, $3 million in 2018 and $7 million in 2017.

[2] Includes performance share units, deferred stock units and historically granted restricted stock.

[3] 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.

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