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]
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