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Equity method investments (Details) (USD $) In Millions, unless otherwise specified | Jun. 30, 2012 | Dec. 31, 2011 | Jun. 30, 2011 | Nov. 30, 2010 Brazilian transmission lines [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%
Portion of remaining interest sold |  |  |  |  | 0.25 |  |  |
Number of years over which remaining ownership interest in equity method investment will be sold |  |  |  | 4 years |  |  |  |
Total assets of equity method investments | $ 104.4 | $ 111.1 | $ 107.7 |  |  |  |  |
Total long-term debt of equity method investments | 30.3 | 37.1 | 49.6 |  |  |  |  |
Investment in equity method investments | 7.4 | 9.2 | 11.4 |  |  |  |  |
Undistributed earnings of equity method investments | $ 2.3 | $ 3.7 | $ 2.1 |  |  |  |  |

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