Fair Value Measurements Fair Value Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Level 1 to level 2 asset transfers, amount	$ 0	$ 0
Level 2 to level 1 assets, transfers, amount	0	0
Level 1 to level 2 liabilities transfers, amount	0	0
Level 2 to level 1 liabilities, 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	6,650	6,392
Rabbi trust investments	16,477	10,522
Derivative liability (1)		(5,428)	[1]
Total	23,127	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	6,650	6,392
Rabbi trust investments	16,477	10,522
Derivative liability (1)		0	[1]
Total	23,127	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)		(5,428)	[1]
Total	0	(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]
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]
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.