Related Party Transactions (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012
Related Party Transaction [Line Items]
Deferred Purchased Power Costs	$ 5,200,000
Transmission costs deferred as regulatory liability	11,000,000
Operating revenues
Electric	145,102,000		136,917,000	125,344,000
Related Party Transaction, Utilities Operating Expense, Purchased Power net of deferred costs	430,666,000	[1]
Operating expenses
Purchased power	425,471,000		416,173,000	405,016,000
Transmission expense	43,876,000		42,460,000	44,942,000
Natural gas purchased for resale	90,000		97,000	116,000
Other operating expenses - paid to Xcel Energy Services Inc.	84,224,000		61,531,000	54,137,000
Interest expense	30,000		22,000	22,000
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	31,000		1,595,000
Accounts payable	26,524,000		24,986,000
NSP-Minnesota
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0		0
Accounts payable	17,333,000		18,584,000
PSCo
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0		0
Accounts payable	22,000		8,000
SPS
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	31,000		26,000
Accounts payable	0		0
Other subsidiaries of Xcel Energy Inc.
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0		1,569,000
Accounts payable	$ 9,169,000		$ 6,394,000

[1] Pursuant to orders issued by the PSCW in December 2013 and February 2014, the 2014 amounts do not reflect $5.2 million of purchased power expenses deferred as a regulatory asset and $11.0 million of transmission costs deferred as a regulatory liability billed to NSP-Wisconsin through the Interchange Agreement from NSP-Minnesota.