Equity Incentive Plans - Changes in performance stock awards (Details) - Performance stock awards - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock units activity
Outstanding at the beginning of the period (in shares)	1,181
Granted (in shares)	282
Adjustment for performance achievement (in shares)	408
Vested (in shares)	(816)
Forfeited (in shares)	(104)
Outstanding at the end of the period (in shares)	951	1,181
Weighted average grant date fair value
Weighted average grant date fair value, nonvested at the beginning of the period (in dollars per share)	$ 87.78
Weighted average grant date fair value, granted (in dollars per share)	123.48	$ 92.49	$ 92.88
Weighted average grant date fair value, adjustment for performance achievement (in dollars per share)	78.49
Weighted average grant date fair value, vested (in dollars per share)	78.49
Weighted average grant date fair value, forfeited (in dollars per share)	101.10
Weighted average grant date fair value, nonvested at the end of the period (in dollars per share)	$ 100.89	$ 87.78
Risk-free rate, high end of the range (as a percent)	1.40%
Expected volatility rate (as a percent)	16.90%
Weighted average volatility (as a percent)	35.10%
Weighted average expected term	2 years 10 months 24 days
Total fair value of restricted stock and restricted stock units vested	$ 101	$ 65	$ 15