Property and equipment: Property and equipment exchange agreement (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property and equipment
Gain (Loss) on Exchange Agreement	$ 7,739	$ 8,156	$ 11,031
Equipment
Property and equipment
Gain (Loss) on Exchange Agreement	7,700	5,300	10,800
Equipment | Level 3
Property and equipment
Fair market value of equipment received in exchange	$ 15,500	$ 17,900	$ 23,100