Related Party Transactions (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating revenues [Abstract]
Electric	$ 125,344	$ 124,334	$ 116,312
Operating expenses [Abstract]
Purchased power, affiliates	405,016	399,649	377,518
Transmission expense	44,942	40,870	38,558
Natural gas purchased for resale	116	98	163
Other operating expenses - paid to Xcel Energy Services Inc.	54,137	54,885	52,826
Interest expense	22	48	56
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	586	0
Accounts payable	31,556	23,285
NSP Minnesota [Member]
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0	0
Accounts payable	26,632	18,003
PSCo [Member]
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0	0
Accounts payable	71	112
SPS [Member]
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0	0
Accounts payable	4	0
Other Subsidiaries of Xcel Energy Inc. [Member]
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	586	0
Accounts payable	$ 4,849	$ 5,170