Risk Management and Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Fair value of derivative financial instruments
Total derivatives assets	$ 12	$ 2
Current liabilities	6,171	4,769
Non-current liabilities and deferred credits	6,028	5,081
Total derivatives liabilities	12,199	9,850
Non-designated as hedging instruments due to regulatory accounting | Commodity contracts
Fair value of derivative financial instruments
Non-current assets and deferred charges- Other	12	2
Non-designated as hedging instruments due to regulatory accounting | Commodity contracts | Gas
Fair value of derivative financial instruments
Current liabilities	1,097	967
Non-current liabilities and deferred credits	3	86
Non-designated as hedging instruments due to regulatory accounting | Commodity contracts | Electric
Fair value of derivative financial instruments
Current liabilities	5,074	3,802
Non-current liabilities and deferred credits	$ 6,025	$ 4,995