Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses [Abstract]
Interest expense	$ 0.2	$ 0.6	$ 0.0
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	4.2	10.5
Accounts payable	20.4	19.9
NSP-Minnesota
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	4.2	4.7
Accounts payable	0.0	0.0
PSCo
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0.0	0.0
Accounts payable	0.4	0.7
Other Subsidiaries of Xcel Energy Inc.
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0.0	5.8
Accounts payable	20.0	19.2
Purchased Power [Member]
Operating expenses [Abstract]
Costs and Expenses, Related Party	0.0	0.0	1.4
Other Expense [Member]
Operating expenses [Abstract]
Costs and Expenses, Related Party	$ 192.0	$ 195.1	$ 196.6