Equity Incentive Plans - Option grant assumptions (Details) - Stock options	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assumptions used to determine fair value of options granted
Weighted average expected term	5 years 9 months 18 days	5 years 8 months 12 days	6 years 1 month 6 days
Expected volatility, low end of the range (as a percent)	15.60%	15.60%	15.70%
Expected volatility, high end of the range (as a percent)	28.90%	30.70%	32.70%
Weighted average volatility (as a percent)	18.40%	19.80%	21.00%
Expected dividends, minimum range (as a percent)	1.90%	1.50%	1.40%
Expected dividends, maximum range (as a percent)	2.20%	2.20%	1.90%
Weighted average expected dividends (as a percent)	2.20%	2.00%	1.90%
Risk-free rate, low end of the range (as a percent)	1.30%	1.30%	0.50%
Risk-free rate, high end of the range (as a percent)	2.70%	3.20%	2.50%