Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses [Abstract]
Purchased power	$ 0.0	$ 1.4	$ 8.8
Other operating expenses - paid to Xcel Energy Services Inc.	195.1	196.6	188.2
Interest expense	0.6	0.0	$ 0.2
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	10.5	1.3
Accounts payable	19.9	22.6
NSP-Minnesota
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	4.7	1.0
Accounts payable	0.0	0.0
PSCo
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0.0	0.0
Accounts payable	0.7	0.3
Other Subsidiaries of Xcel Energy Inc.
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	5.8	0.3
Accounts payable	$ 19.2	$ 22.3