Property and equipment: Property and equipment addiional information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property and equipment
Depreciation and amortization expense	$ 70,527	$ 69,481	$ 64,358
Capitalized salaries and related benefits of employees	8,300	7,600	7,400
Gain (Loss) on Asset Exchange Transactions	8,156	11,031	934
Equipment
Property and equipment
Gain (Loss) on Asset Exchange Transactions	5,300	10,800	900
Equipment | Level 3
Property and equipment
Fair market value of equipment received in exchange	17,900	23,100	2,000
Property, equipment and capital leases
Property and equipment
Depreciation and amortization expense	$ 70,500	$ 69,500	$ 64,300