Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) (Commodity Derivatives, Net [Member], USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Commodity Derivatives, Net [Member]
Changes in Level 3 Commodity Derivatives [Roll Forward]
Balance at beginning of period	$ 7,642,000		$ 5,324,000		$ 16,649,000		$ 12,417,000
Purchases	51,386,000		37,296,000		51,386,000		37,297,000
Settlements	(8,503,000)		(12,675,000)		(20,952,000)		(21,560,000)
Net transactions recorded during the period:
Losses recognized in earnings	(217,000)	[1]	0	[1]	(279,000)	[1]	(9,000)	[1]
(Losses) gains recognized as regulatory assets and liabilities	(3,090,000)		3,844,000		414,000		5,644,000
Balance at end of period	47,218,000		33,789,000		47,218,000		33,789,000
Transfers into Level 3	0		0		0		0
Transfers out of Level 3	$ 0		$ 0		$ 0		$ 0

[1] These amounts relate to commodity derivatives held at the end of the period.