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Equity method investments (Details) (USD $) In Millions, unless otherwise specified | Jun. 30, 2013 | Dec. 31, 2012 | Jun. 30, 2012 | Aug. 31, 2012 ECTE [Member] | Nov. 30, 2011 ECTE [Member] | Nov. 30, 2010 ECTE [Member] | Nov. 30, 2010 ENTE [Member] | Nov. 30, 2010 ERTE [Member]
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Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Percentage ownership interest in equity method investment sold |  |  |  |  |  | 59.96% | 100.00% | 100.00%
Number of years over which remaining ownership interest in equity method investment will be sold |  |  |  |  |  | 4 years |  |
Portion of remaining interest sold |  |  |  | 25.00% | 25.00% |  |  |
Total assets of equity method investments | $ 114.8 | $ 129.0 | $ 104.4 |  |  |  |  |
Total long-term debt of equity method investments | 56.2 | 65.5 | 30.3 |  |  |  |  |
Investment in equity method investments | 5.5 | 6.9 | 7.4 |  |  |  |  |
Undistributed earnings of equity method investments | $ 2.0 | $ 3.4 | $ 2.3 |  |  |  |  |

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- Definition This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized. + References 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 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=16385135&loc=d3e33749-111570 + Details
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