Related Party Transactions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenues
Electric	$ 177,234	$ 170,483	$ 163,255
Operating expenses
Purchased power	421,609	413,615	419,028
Transmission expense	68,613	61,920	54,070
Natural gas purchased for resale	47	41	45
Other operating expenses - paid to Xcel Energy Services Inc.	92,715	106,454	93,890
Interest expense	7	4	$ 2
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	3,391	333
Accounts payable	29,628	29,037
NSP-Minnesota
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0	0
Accounts payable	17,825	18,567
PSCo
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0	0
Accounts payable	61	974
SPS
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0	333
Accounts payable	7	0
Other subsidiaries of Xcel Energy Inc.
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	3,391	0
Accounts payable	$ 11,735	$ 9,496