Derivative instruments: Master netting agreements (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Natural gas swaps
Assets:
Gross Amounts of Recognized Assets (Liabilities)	$ (22,848)	$ (18,914)
Net Amounts of Assets Presented on the Balance Sheet	(22,848)	(18,914)
Interest rate options
Assets:
Gross Amounts of Recognized Assets (Liabilities)	26,925	53,603
Gross Amounts offset on the Balance Sheets	(25,915)	(49,232)
Net Amounts of Assets Presented on the Balance Sheet	$ 1,010	$ 4,371

Fair value, after effects of master netting arrangements, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset offset against an obligation to return collateral. Includes assets not subject to a master netting arrangement and not elected to be offset.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35706170&loc=d3e41678-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226016-175313

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