Description of the business and summary of significant accounting policies: Revenue recognition - Excise taxes (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Description of the business and summary of significant accounting policies:
Excise taxes and surcharge	$ 10.9	$ 9.1	$ 3.6

The amount of excise and sales taxes included in sales and revenues, which are then deducted as a cost of sales. Includes excise taxes, which are applied to specific types of transactions or items (such as gasoline or alcohol); and sales, use and value added taxes, which are applied to a broad class of revenue-producing transactions involving a wide range of goods and services.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 45
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=68080631&loc=d3e61097-111654

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration