Related Party Transactions (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating revenues
Electric	$ 9,614	$ 8,136	$ 9,271
Other	4,441	4,441	4,441
Operating expenses
Purchased power	23	1,331	6,539
Other operating expenses - paid to Xcel Energy Services Inc.	454,250	375,766	316,548
Interest expense	158	132	114
Interest income	61	273	122
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	50,842	19,136
Accounts payable	46,736	45,902
NSP-Minnesota
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0	18,065
Accounts payable	6,706	0
NSP-Wisconsin
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	22	8
Accounts payable	0	0
SPS
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	5,803	1,056
Accounts payable	0	0
Other subsidiaries of Xcel Energy Inc.
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	45,017	7
Accounts payable	$ 40,030	$ 45,902