Related Party Transactions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating revenues
Electric	$ 8,632	$ 9,614	$ 8,136
Other	4,441	4,441	4,441
Operating expenses
Purchased power	0	23	1,331
Other operating expenses - paid to Xcel Energy Services Inc.	414,620	454,250	375,766
Interest expense	211	158	132
Interest income	45	61	$ 273
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	4,909	50,842
Accounts payable	76,643	46,736
NSP-Minnesota
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	4,419	0
Accounts payable	0	6,706
NSP-Wisconsin
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	71	22
Accounts payable	0	0
SPS
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	414	5,803
Accounts payable	0	0
Other subsidiaries of Xcel Energy Inc.
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	5	45,017
Accounts payable	$ 76,643	$ 40,030