Fair Value Measurements Unobservable Input Reconciliation (Details) (USD $)	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ (35,526,000)		$ (95,352,000)
Included in earnings	1,598,000	[1]	(21,947,000)	[1]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Regulatory Assets (Liabilities)	674,000		(1,283,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	5,651,000	[2]	20,824,000	[2]
Transferred into Level 3	(7,700,000)		(16,874,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	7,327,000		4,101,000
Balance at end of period	(27,976,000)		(110,531,000)
Electric portfolio [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	(33,924,000)		(90,311,000)
Included in earnings	1,598,000	[1]	(21,947,000)	[1]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Regulatory Assets (Liabilities)	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	6,384,000	[2]	21,042,000	[2]
Transferred into Level 3	(7,700,000)		(16,874,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	7,200,000		726,000
Balance at end of period	(26,442,000)		(107,364,000)
Unrealized (gain) loss on derivatives instuments, net	(1,400,000)
Natural gas portfolio [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	(1,602,000)		(5,041,000)
Included in earnings	0	[1]	0	[1]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Regulatory Assets (Liabilities)	674,000		(1,283,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	(733,000)	[2]	(218,000)	[2]
Transferred into Level 3	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	127,000		3,375,000
Balance at end of period	$ (1,534,000)		$ (3,167,000)

[1] Income Statement location: Unrealized (gain) loss on derivative instruments, net. Includes unrealized gains (losses) on derivatives still held in position as of the reporting date for electric derivatives of $1.4 million and $(19.0) million for the three months ended March 31, 2013 and 2012, respectively.

[2] The Company had no purchases, sales or issuances during the reported periods.