Risk Management and Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair value of derivative financial instruments
Total derivatives assets	$ 901	$ 2
Current liabilities	3,549	4,769
Non-current liabilities and deferred credits	3,277	5,081
Total derivatives liabilities	6,826	9,850
Non-designated as hedging instruments due to regulatory accounting | Commodity contracts | Gas
Fair value of derivative financial instruments
Current assets	8
Non-current assets and deferred charges- Other	38	2
Current liabilities	190	967
Non-current liabilities and deferred credits		86
Non-designated as hedging instruments due to regulatory accounting | Commodity contracts | Electric
Fair value of derivative financial instruments
Current assets	409
Non-current assets and deferred charges- Other	446
Current liabilities	3,359	3,802
Non-current liabilities and deferred credits	$ 3,277	$ 4,995