Related Party Transactions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating revenues
Electric	$ 8,809	$ 8,632	$ 9,614
Other	4,525	4,441	4,441
Operating expenses
Purchased power	56	0	23
Other operating expenses - paid to Xcel Energy Services Inc.	446,086	414,620	454,250
Interest expense	149	211	158
Interest income	0	45	$ 61
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	9,421	4,909
Accounts payable	98,797	76,643
NSP-Minnesota
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	7,669	4,419
Accounts payable	0	0
NSP-Wisconsin
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	974	71
Accounts payable	0	0
SPS
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	745	414
Accounts payable	0	0
Other subsidiaries of Xcel Energy Inc.
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	33	5
Accounts payable	$ 98,797	$ 76,643