Stock option and award plan: Incentive award plan - activity (Details) - Stock options $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Stock option activity
Outstanding at the beginning of the period (in shares) | shares	183,322
Granted (in shares) | shares	81,236
Cancelled and expired (in shares) | shares	(35,340)
Exercised (in shares) | shares	(39,289)
Outstanding at the end of the period (in shares) | shares	189,929
Exercisable at the end of the period (in shares) | shares	109,084
Expected to vest (in shares) | shares	168,762
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 32.35
Granted (in dollars per share) | $ / shares	43.93
Cancelled and expired (in dollars per share) | $ / shares	38.72
Exercised (in dollars per share) | $ / shares	31.12
Outstanding at the end of the period (in dollars per share) | $ / shares	36.37
Exercisable at the end of the period (in dollars per share) | $ / shares	32.85
Expected to vest (in dollars per share) | $ / shares	$ 35.63
Stock option activity parenthetical
Exercised - intrinsic value | $	$ 0.5
Exercised - cash received | $	1.2
Outstanding end of period - intrinsic value | $	$ 1.8
Outstanding - weighted-average term	7 years 6 months
Exercisable - intrinsic value | $	$ 1.4
Exercisable - weighted average term	6 years 4 months 24 days
Expected to vest - intrinsic value | $	$ 1.7
Expected to vest - weighted average term	7 years 3 months 18 days