Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses
Interest expense	$ 0.7	$ 0.3	$ 0.0
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	43.8	11.0
Accounts payable	76.0	109.7
NSP-Wisconsin
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	7.9	11.0
Accounts payable	0.0	0.0
PSCo
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0.0	0.0
Accounts payable	18.8	17.9
SPS
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0.0	0.0
Accounts payable	4.2	4.7
Other subsidiaries of Xcel Energy Inc.
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	35.9	0.0
Accounts payable	53.0	87.1
Purchased Power
Operating expenses
Costs and Expenses, Related Party	60.5	61.1	66.8
Transmission Expense
Operating expenses
Costs and Expenses, Related Party	116.2	96.8	110.5
Other Expense
Operating expenses
Costs and Expenses, Related Party	533.2	534.8	539.4
Electricity, US Regulated
Operating revenues
Operating Revenue from Related Parties	457.4	473.7	490.2
Natural Gas, US Regulated
Operating revenues
Operating Revenue from Related Parties	$ 0.5	$ 0.0	$ 0.0