Income taxes: Temporary differences (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Tax Assets:
Net operating loss carry-forwards	$ 246,276	$ 273,999
Tax credits	2,119	1,965
Equity-based compensation	3,976	4,901
Operating leases	40,627	40,081
Total gross deferred tax assets	292,998	320,946
Valuation allowance	(132,800)	(150,589)
Net deferred tax assets	160,198	170,357
Deferred Tax Liabilities:
Depreciation and amortization	46,642	37,364
Accrued liabilities and other	103,705	105,554
Right-of-use assets	37,784	37,097
Gross deferred tax liabilities	188,131	180,015
Net deferred tax liabilities	$ 27,933	$ 9,658