Equity method investments (Details) (USD $)			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011 Brazilian transmission lines [Member]	Dec. 31, 2010 Brazilian transmission lines [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]
Schedule of equity method investments [Line Items]
Recognized gain on sale of ownership interests			$ 1,000,000	$ 22,700,000
Recognized gain on sale of ownership interests, after tax			600,000	13,800,000
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
Remaining interest					25.00%	25.00%
Total assets of equity method investments	129,000,000	111,100,000
Total long-term debt of equity method investments	65,500,000	37,100,000
Investment in equity method investments	6,900,000	9,200,000
Undistributed earnings of equity method investments	$ 3,400,000	$ 3,700,000