Equity method investments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 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]
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	$ 105.3	$ 111.1	$ 108.2
Total long-term debt of equity method investments	35.9	37.1	46.3
Investment in equity method investments	8.2	9.2	11.7
Undistributed earnings of equity method investments	$ 2.2	$ 3.7	$ 2.4