Stock option and award plan: Non-vested awards, tables (Details) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Number of shares
Non-vested at the beginning of the period (in shares)	1,282,646	1,039,323
Non-vested at the end of the period (in shares)	870,751	1,282,646	1,039,323
Restricted stock
Number of shares
Non-vested at the beginning of the period (in shares)	1,282,646
Granted (in shares)	62,900	800,000	200,000
Vested (in shares)	(427,090)
Forfeited (in shares)	(47,705)
Non-vested at the end of the period (in shares)	870,751	1,282,646
Weighted-Average Grant Date Fair Value
Non-vested at the beginning of the period (in dollars per share)	$ 28.26
Granted (in dollars per share)	33.19	$ 34.57	$ 27.82
Vested (in dollars per share)	20.53
Forfeited (in dollars per share)	31.14
Non-vested at the end of the period (in dollars per share)	$ 32.25	$ 28.26