Income taxes: Temporary differences (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets:
Net operating loss carry-forwards	$ 271,219	$ 273,509
Tax credits	2,771	2,237
Equity-based compensation	2,327	2,279
Total gross deferred tax assets	276,317	278,025
Deferred Tax Liabilities:
Depreciation and amortization	22,777	29,984
Accrued liabilities and other	106,251	95,606
Total gross deferred tax liabilities	129,028	125,590
Deferred tax assets (liabilities)
Net deferred tax assets before valuation allowance	147,289	152,435
Valuation allowance	(129,673)	(110,194)
Net deferred tax assets	$ 17,616	$ 42,241