Borrowings and Other Financing Instruments, Intercompany Borrowing Arrangement and Other Short-Term Borrowings (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Short-term Debt [Line Items]
Notes payable to affiliates	$ 470	$ 550
Intercompany Borrowing Arrangement
Short-term Debt [Line Items]
Notes payable to affiliates	$ 500	$ 600
Weighted average interest rate at end of period (in hundredths)	0.27%	0.33%