Equity method investments (Details) (Brazilian Transmission Lines [Member], USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010
Brazilian Transmission Lines [Member]
Schedule of Equity Method Investments [Line Items]
Number of separate parties with which company entered into agreements to sell equity ownership interests	3
Ownership interest in ECTE sold (in hundredths)	59.96%
Number of years over which remaining ownership interest in ECTE will be sold (in years)	4
Total assets of equity method investments	$ 107.7	$ 107.4	$ 369.9
Total long-term debt of equity method investments	49.6	30.1	157.1
Investment in equity method investments	11.4	10.9	57.9
Undistributed earnings of equity method investments	$ 2.1	$ 1.9	$ 11.1