Related Party Transactions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenues [Abstract]
Electric	$ 2	$ 56	$ 0
Operating expenses [Abstract]
Purchased power	1,436	8,809	8,632
Other operating expenses - paid to Xcel Energy Services Inc.	196,558	188,175	197,134
Interest expense	0	189	156
Interest income	0	0	$ 6
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	1,297	949
Accounts payable	22,577	14,414
NSP-Minnesota
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	964	935
Accounts payable	0	0
NSP-Wisconsin
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	7	0
Accounts payable	0	333
PSCo
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0	0
Accounts payable	279	745
Other Subsidiaries of Xcel Energy Inc.
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	326	14
Accounts payable	$ 22,298	$ 13,336