Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	$ 0.3	$ 15.1
Accounts payable	19.0	20.0
NSP-Minnesota
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0.0	0.0
Accounts payable	7.9	11.0
PSCo
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0.3	0.2
Accounts payable	0.0	0.0
Other subsidiaries of Xcel Energy Inc.
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0.0	14.9
Accounts payable	11.1	9.0
Purchased Power
Operating expenses
Costs and Expenses, Related Party	393.9	410.9	$ 421.6
Transmission Expense
Operating expenses
Costs and Expenses, Related Party	63.5	62.8	68.6
Natural Gas Purchase for Resale
Operating expenses
Costs and Expenses, Related Party	0.5	0.0	0.0
Other Expense
Operating expenses
Costs and Expenses, Related Party	$ 86.8	$ 86.9	$ 92.7