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Equity method investments (Details) (USD $)		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011 Brazilian transmission lines [Member]	Aug. 31, 2013 ECTE [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]
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			25.00%	25.00%	25.00%
Recognized gain on sale of ownership interests		$ 1,000,000
Recognized gain on sale of ownership interests, after tax		600,000
Total assets of equity method investments	129,000,000
Total long-term debt of equity method investments	65,500,000
Investment in equity method investments	6,900,000
Undistributed earnings of equity method investments	$ 3,400,000

X

- Definition

This item represents the amount of the after-tax gain (loss) arising from the disposal of an equity method investment.

+ References

No definition available.

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X

- Definition

The amount of long-term debt reported by an equity method investment of the entity.

+ References

No definition available.

+ Details

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- Definition

Number of years over which remaining ownership interest in equity method investment will be sold.

+ References

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- Definition

Percentage ownership interest in equity method investment sold.

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Amount of remaining ownership interest sold.

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- Definition

Undistributed earnings of equity method investments.

+ References

No definition available.

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- Definition

Amount of gain (loss) on sale or disposal of an equity method investment.

+ References

No definition available.

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- Definition

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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- Definition

The amount of assets reported by an equity method investment of the entity.

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