Fair Value of Financial Assets and Liabilities, Impact of Derivative Activity (Details) (Other Derivative Instruments [Member], Natural Gas Commodity [Member])	Mar. 31, 2013 MMBTU		Dec. 31, 2012 MMBTU
Other Derivative Instruments [Member] | Natural Gas Commodity [Member]
Gross Notional Amounts of Commodity Forwards and Options [Abstract]
Notional amount	0	[1],[2]	53,000	[1],[2]

[1] Amounts are not reflective of net positions in the underlying commodities.

[2] Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.