Stock option and award plan: Non-vested restricted stock (Details) - Restricted stock - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Number of shares
Non-vested at the beginning of the period (in shares)	1,339,596
Granted (in shares)	471,080	500,000	500,000
Vested (in shares)	(509,919)
Forfeited (in shares)	(47,436)
Non-vested at the end of the period (in shares)	1,253,321	1,339,596
Weighted-Average Grant Date Fair Value
Non-vested at the beginning of the period (in dollars per share)	$ 55.70
Granted (in dollars per share)	64.59	$ 75.18	$ 53.53
Vested (in dollars per share)	49.12
Forfeited (in dollars per share)	57.16
Non-vested at the end of the period (in dollars per share)	$ 61.66	$ 55.70