Stock option and award plan: Incentive award plan, tables (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Incentive award plan
Weighted-average per share grant date fair value (in dollars per share)	$ 6.23	$ 6.29	$ 8.32
Assumptions used for determining the fair value of options granted
Dividend yield (as a percent)	3.70%	3.80%	3.60%
Expected volatility (as a percent)	30.80%	32.50%	37.00%
Risk-free interest rate (as a percent)	1.40%	1.50%	1.60%
Expected life of the option term	4 years 7 months 6 days	4 years 7 months 6 days	4 years 7 months 6 days
Stock option activity
Outstanding at the beginning of the period (in shares)	201,587
Granted (in shares)	72,752
Cancelled and expired (in shares)	(37,772)
Exercised (in shares)	(53,245)
Cash received from exercise of stock option	$ 1.2
Outstanding at the end of the period (in shares)	183,322	201,587
Exercisable at the end of the period	$ 1.2
Exercisable at the end of the period (in shares)	103,898
Expected to vest (in shares)	162,235
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 28.42
Granted (in dollars per share)	37.54
Cancelled and expired (in dollars per share)	34.66
Exercised (in dollars per share)	22.92
Outstanding at the end of the period (in dollars per share)	32.35	$ 28.42
Exercisable at the end of the period (in dollars per share)	29.61
Expected to vest (in dollars per share)	$ 31.85
Weighted-Average Remaining Contractual Life (in years)
Outstanding at the end of the period	7 years 4 months 24 days
Exercisable at the end of the period	6 years 2 months 12 days
Expected to vest	7 years 1 month 6 days
Aggregate Intrinsic Value
Exercised	$ 0.9
Outstanding at the end of the period	1.7
Exercisable at the end of the period	1.2
Expected to vest	$ 1.5