Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating revenues
Related Party Transaction, Electric Domestic Regulated Revenue	$ 157.9	$ 177.2	$ 170.4
Operating expenses
Purchased power	410.9	421.6	413.6
Transmission expense	62.8	68.6	61.9
Other operating expenses - paid to Xcel Energy Services Inc.	86.9	92.7	$ 106.5
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	15.1	3.4
Accounts payable	20.0	29.6
NSP-Minnesota
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0.0	0.0
Accounts payable	11.0	17.8
PSCo
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0.2	0.0
Accounts payable	0.0	0.0
Other subsidiaries of Xcel Energy Inc.
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	14.9	3.4
Accounts payable	$ 9.0	$ 11.8