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Equity method investments (Details) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Sep. 30, 2011
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]
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       0.25 0.25      
Total assets of equity method investments $ 110.6 $ 111.1 $ 108.0          
Total long-term debt of equity method investments 28.2 37.1 39.7          
Investment in equity method investments 7.4 9.2 10.5          
Undistributed earnings of equity method investments $ 4.1 $ 3.7 $ 2.9