Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating revenues
Electric	$ 0.0	$ 1.4	$ 8.8
Other	4.4	4.5	4.5
Operating expenses
Other operating expenses - paid to Xcel Energy Services Inc.	518.7	485.1	446.1
Interest expense	0.0	0.0	$ 0.1
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	80.8	14.7
Accounts payable	46.0	58.7
NSP-Minnesota
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	17.9	7.7
Accounts payable	0.0	0.0
NSP-Wisconsin
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0.0	0.0
Accounts payable	0.2	0.0
SPS
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0.7	0.3
Accounts payable	0.0	0.0
Other subsidiaries of Xcel Energy Inc.
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	62.2	6.7
Accounts payable	$ 45.8	$ 58.7