Cash flow information Cash flow information (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Supplemental Cash Flow Information [Abstract]
Capitalized interest and AFUDC borrowed	$ 2.6	$ 2.4

Amount of interest capitalized, including amount of allowance for funds used during construction.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Allowance for Funds Used during Construction

-URI http://asc.fasb.org/extlink&oid=6504829

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 835

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6501662&loc=d3e56162-110433

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