Summary of significant accounting policies: New accounting pronouncements (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
New Accounting Pronouncements
Deferred costs	$ (93,651,000)	$ (97,423,000)
Long-term debt	$ (7,291,154,000)	(7,015,577,000)
Accounting Standards Update 2015-03 - Simplifying the Presentation of Debt Issuance Costs \| Adjustments for New Accounting Principle, Early Adoption
New Accounting Pronouncements
Deferred costs		97,423,000
Long-term debt		$ 97,423,000

Net amount of long-term deferred finance costs capitalized at the end of the reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28555-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 17
-Article 5

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