Income taxes: - NOLs and alternative minimum tax credits (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Income taxes
NOLs	$ 76,411
Alternative Minimum Tax Credits
Income taxes
Tax Credits	599
2018
Income taxes
NOLs	61,533
2019
Income taxes
NOLs	10,516
2020
Income taxes
NOLs	4,362
None \| Alternative Minimum Tax Credits
Income taxes
Tax Credits	$ 599

The amount of the tax credit carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Carryforwards
-URI http://asc.fasb.org/extlink&oid=6506874

Name:	us-gaap_TaxCreditCarryforwardAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant