Income taxes: Temporary differences comprising our net deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Assets:
Net operating loss carry-forwards	$ 373,655	$ 370,344
Tax credits	2,237	2,317
Equity-based compensation	2,279	1,474
Total gross deferred tax assets	378,171	374,135
Deferred Tax Liabilities:
Depreciation and amortization	29,984	18,038
Accrued liabilities and other	95,606	97,417
Total gross deferred tax liabilities	125,590	115,455
Deferred tax assets (liabilities)
Net deferred tax assets before valuation allowance	252,581	258,680
Valuation allowance	(210,340)	(213,538)
Net deferred tax assets	$ 42,241	$ 45,142