Stock option and award plan: Incentive award plan fair value assumptions (Details) - Stock options - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Incentive award plan
Weighted-average per share grant date fair value (in dollars per share)	$ 12.22	$ 13.21	$ 8.92
Assumptions used for determining the fair value of options granted
Dividend yield (as a percent)	4.60%	3.40%	4.50%
Expected volatility (as a percent)	33.40%	31.50%	28.30%
Risk-free interest rate (as a percent)	0.60%	1.10%	2.50%
Expected life of the option term (in years)	4 years 2 months 12 days	4 years 2 months 12 days	4 years 3 months 18 days