Risk Management and Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair value of derivative financial instruments
Total derivatives assets	$ 206	$ 2
Current liabilities	4,068	4,769
Non-current liabilities and deferred credits	5,734	5,081
Total derivatives liabilities	9,802	9,850
Non-designated as hedging instruments due to regulatory accounting | Commodity contracts | Gas
Fair value of derivative financial instruments
Non-current assets and deferred charges- Other	17	2
Current liabilities	553	967
Non-current liabilities and deferred credits	1	86
Non-designated as hedging instruments due to regulatory accounting | Commodity contracts | Electric
Fair value of derivative financial instruments
Current assets	82
Non-current assets and deferred charges- Other	107
Current liabilities	3,515	3,802
Non-current liabilities and deferred credits	$ 5,733	$ 4,995