Equity Incentive Plans - Option grant assumptions (Details) - Stock options	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Assumptions used to determine fair value of options granted
Weighted average expected term	6 years 1 month 6 days	5 years 9 months 18 days	5 years 8 months 12 days
Expected volatility, low end of the range (as a percent)	16.30%	15.60%	15.60%
Expected volatility, high end of the range (as a percent)	37.10%	28.90%	30.70%
Weighted average volatility (as a percent)	17.60%	18.40%	19.80%
Expected dividends, minimum range (as a percent)	1.60%	1.90%	1.50%
Expected dividends, maximum range (as a percent)	2.40%	2.20%	2.20%
Weighted average expected dividends (as a percent)	1.80%	2.20%	2.00%
Risk-free rate, low end of the range (as a percent)	0.10%	1.30%	1.30%
Risk-free rate, high end of the range (as a percent)	1.80%	2.70%	3.20%