Equity Incentive Plans - Option grant assumptions (Details) - Stock options	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assumptions used to determine fair value of options granted
Weighted average expected term	6 years 1 month 6 days	5 years	6 years 6 months
Expected volatility, low end of the range (as a percent)	15.70%	16.00%	16.00%
Expected volatility, high end of the range (as a percent)	32.70%	34.30%	37.80%
Weighted average volatility (as a percent)	21.00%	24.30%	24.70%
Expected dividends, minimum range (as a percent)	1.40%	1.90%	1.60%
Expected dividends, maximum range (as a percent)	1.90%	2.10%	2.10%
Weighted average expected dividends (as a percent)	1.90%	2.10%	1.70%
Risk-free rate, low end of the range (as a percent)	0.50%	0.20%	0.00%
Risk-free rate, high end of the range (as a percent)	2.50%	2.40%	2.40%