Income taxes: - Other tax information (Details)	12 Months Ended
Dec. 31, 2016 USD ($)
Income taxes
Uncertain tax positions	$ 0
State jurisdictions \| Maximum
Income taxes
Period during which state impact of any federal changes remains subject to examination by various states	1 year

The noncurrent portion of the amount recognized for uncertain tax positions as of the balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.24)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_LiabilityForUncertainTaxPositionsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant