Fair Value Measurements Unobservable Input Reconciliation (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ (110,531,000)		$ (91,543,000)		$ (95,352,000)		$ (91,295,000)
Included in earnings	5,077,000	[1]	(4,244,000)	[1]	(16,870,000)	[2]	(19,951,000)	[2]
Included in regulatory assets / liabilities	(4,000)		1,860,000		(1,287,000)		2,979,000
Settlements	14,696,000	[3]	7,107,000	[3]	35,520,000	[3]	17,547,000	[3]
Transferred into Level 3	(38,971,000)		363,000		(55,845,000)		363,000
Transferred out of Level 3	46,461,000		4,920,000		50,562,000		8,820,000
Balance at end of period	(83,272,000)		(81,537,000)		(83,272,000)		(81,537,000)
Electric portfolio [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	(107,364,000)		(89,758,000)		(90,311,000)		(87,436,000)
Included in earnings	5,077,000	[1]	(4,244,000)	[1]	(16,870,000)	[2]	(19,951,000)	[2]
Included in regulatory assets / liabilities	0		0		0		0
Settlements	14,372,000	[3]	8,234,000	[3]	35,415,000	[3]	19,139,000	[3]
Transferred into Level 3	(38,674,000)		0		(55,548,000)		0
Transferred out of Level 3	46,136,000		4,475,000		46,861,000		6,955,000
Balance at end of period	(80,453,000)		(81,293,000)		(80,453,000)		(81,293,000)
Unrealized (gain) loss on derivatives instuments, net	(6,200,000)		5,000,000		12,800,000		(17,500,000)
Natural gas portfolio [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	(3,167,000)		(1,785,000)		(5,041,000)		(3,859,000)
Included in earnings	0	[1]	0	[1]	0	[2]	0	[2]
Included in regulatory assets / liabilities	(4,000)		1,860,000		(1,287,000)		2,979,000
Settlements	324,000	[3]	(1,127,000)	[3]	105,000	[3]	(1,592,000)	[3]
Transferred into Level 3	(297,000)		363,000		(297,000)		363,000
Transferred out of Level 3	325,000		445,000		3,701,000		1,865,000
Balance at end of period	$ (2,819,000)		$ (244,000)		$ (2,819,000)		$ (244,000)

[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 $6.2 million and $(5.0) million for the three months ended June 30, 2012 and 2011, respectively. 2 Income Stat

[2] 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 $(12.8) million and $(17.5) million for the six months ended June 30, 2012 and 2011, respectively. 3 The Company h

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