Fair Value Measurements Fair Value Recurring Basis (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Total Net Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Restricted cash	$ 6,392		$ 12,292
Rabbi trust investments	10,522		8,049
Derivative liability (1)	(5,428)	[1]	(20,312)	[1]
Total	11,486		29
Quoted Prices In Active Markets for Identical Assets or Liabilities, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Restricted cash	6,392		12,292
Rabbi trust investments	10,522		8,049
Derivative liability (1)	0	[1]	0	[1]
Total	16,914		20,341
Significant Other Observable Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Restricted cash	0		0
Rabbi trust investments	0		0
Derivative liability (1)	(5,428)	[1]	(20,312)	[1]
Total	(5,428)		(20,312)
Significant Unobservable Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Restricted cash	0		0
Rabbi trust investments	0		0
Derivative liability (1)	0	[1]	0	[1]
Total	0		0
Margin Cash Collateral Offset [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Restricted cash	0		0
Rabbi trust investments	0		0
Derivative liability (1)	0	[1]	0	[1]
Total	$ 0		$ 0

[1] The changes in the fair value of these derivatives are deferred as a regulatory asset or liability until the contracts are settled. Upon settlement, associated proceeds or costs are passed through the applicable cost tracking mechanism to customers.