Stock option and award plan: Non-vested restricted stock (Details) - Restricted stock - $ / shares	1 Months Ended	12 Months Ended
	May 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of shares
Non-vested at the beginning of the period (in shares)		908,761
Granted (in shares)	73,450	499,654	400,000	100,000
Vested (in shares)		(286,083)
Forfeited (in shares)		(10,181)
Non-vested at the end of the period (in shares)		1,112,151	908,761
Weighted-Average Grant Date Fair Value
Non-vested at the beginning of the period (in dollars per share)		$ 35.94
Granted (in dollars per share)		40.52	$ 38.87	$ 33.19
Vested (in dollars per share)		35.35
Forfeited (in dollars per share)		39.88
Non-vested at the end of the period (in dollars per share)		$ 38.11	$ 35.94