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Equity method investments (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2013 | Aug. 31, 2013 ECTE [Member] | 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% | 25.00% |  |  |
Total assets of equity method investments | $ 142.9 |  |  |  |  |  |
Total long-term debt of equity method investments | 63.9 |  |  |  |  |  |
Investment in equity method investments | 5.7 |  |  |  |  |  |
Undistributed earnings of equity method investments | $ 2.2 |  |  |  |  |  |

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