Stock option and award plan: Incentive award plan fair value assumptions (Details) - Stock options - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Incentive award plan
Weighted-average per share grant date fair value (in dollars per share)	$ 7.06	$ 6.23	$ 6.29
Assumptions used for determining the fair value of options granted
Dividend yield (as a percent)	4.10%	3.70%	3.80%
Expected volatility (as a percent)	27.10%	30.80%	32.50%
Risk-free interest rate (as a percent)	2.00%	1.40%	1.50%
Expected life of the option term (in years)	4 years 6 months	4 years 7 months 6 days	4 years 7 months 6 days