Fair Value Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value, assets, level 1 to level 2 transfers, amount	$ 0		$ 0
Fair value, assets, level 2 to level 1 transfers, amount	0		0
Fair value, liabilities, level 1 to level 2 transfers, amount	0		0
Fair value, liabilities, level 2 to level 1 transfers, amount	0		0
Fair Value, Measurements, Recurring [Member] | Total Net Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Restricted cash	7,987		6,392
Rabbi trust investments	16,835		10,522
Derivative liability (1)	(653)	[1]	(5,428)	[1]
Total	24,169		11,486
Fair Value, Measurements, Recurring [Member] | 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	7,987		6,392
Rabbi trust investments	16,835		10,522
Derivative liability (1)	0	[1]	0
Total	24,822		16,914
Fair Value, Measurements, Recurring [Member] | 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)	(653)	[1]	(5,428)
Total	(653)		(5,428)
Fair Value, Measurements, Recurring [Member] | 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
Total	0		0
Fair Value, Measurements, Recurring [Member] | 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
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.