Related Party Transactions (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating revenues [Abstract]
Electric	$ 23	$ 1,331	$ 6,539
Operating expenses [Abstract]
Purchased power	9,614	8,136	9,271
Other operating expenses - paid to Xcel Energy Services Inc.	145,917	127,669	117,277
Interest expense	73	178	76
Interest income	3	0	10
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	1,983	15,840
Accounts payable	19,790	15,387
NSP-Minnesota
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	1,983	3,462
Accounts payable	0	0
NSP-Wisconsin
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0	0
Accounts payable	31	26
PSCo
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0	0
Accounts payable	5,803	1,056
Other Subsidiaries of Xcel Energy Inc.
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0	12,378
Accounts payable	$ 13,956	$ 14,305