Stock Plans - Performance Share Units and Other Stock-Based Awards Activity (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Number of Shares
Balance at January 1, 2018 (in shares) | shares	7,669,705
Shares granted (in shares) | shares	1,940,823
Vested (in shares) | shares	(2,084,527)	[1]
Forfeited (in shares) | shares	(195,028)	[1]
Balance at March 31, 2018 (in shares) | shares	7,330,973
Weighted-average grant date fair value per share
Balance at January 1, 2018 (in dollars per share) | $ / shares	$ 39.74
Shares granted (in dollars per share) | $ / shares	47.47
Vested (in dollars per share) | $ / shares	38.28	[1]
Forfeited (in dollars per share) | $ / shares	38.80	[1]
Balance at March 31, 2018 (in dollars per share) | $ / shares	$ 42.23
Weighted-Average Aggregate Fair Value
Weighted average grant date fair value of shares granted | $	$ 92
Weighted average grant date fair value of shares vested | $	80	[1]
Maximum
Weighted-Average Aggregate Fair Value
Actual tax benefit realized for the tax deductions from the shares vested | $	$ 4
Annual grant to eligible employees
Grant Date
Grant Date	Feb. 22, 2018
Performance share units
Number of Shares
Shares granted (in shares) | shares	1,048,770
Weighted-average grant date fair value per share
Shares granted (in dollars per share) | $ / shares	$ 51.23
Deferred stock units
Number of Shares
Shares granted (in shares) | shares	788,310
Weighted-average grant date fair value per share
Shares granted (in dollars per share) | $ / shares	$ 43.51
Additional shares granted
Number of Shares
Shares granted (in shares) | shares	103,743	[2]
Weighted-average grant date fair value per share
Shares granted (in dollars per share) | $ / shares	$ 39.48	[2]

[1] Includes performance share units, deferred stock units and historically granted restricted stock. The actual tax benefit realized and recorded in the provision for income taxes for the tax deductions from the shares vested totaled $4 million in the three months ended March 31, 2018.

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