Regulatory Assets and Liabilities - Regulatory Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Regulatory liabilities
Other current liabilities	$ 14.3	$ 51.0
Regulatory liabilities	3,946.0	3,928.1
Total regulatory liabilities	3,960.3	3,979.1
Income tax related items
Regulatory liabilities
Total regulatory liabilities	1,998.5	2,137.7
Removal costs
Regulatory liabilities
Total regulatory liabilities	1,248.0	1,221.1
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	397.3	378.1
Derivatives
Regulatory liabilities
Total regulatory liabilities	124.1	16.4
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	84.2	78.5
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	37.1	25.5
Earnings sharing mechanisms
Regulatory liabilities
Total regulatory liabilities	28.4	36.9
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	13.7	59.9
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 29.0	$ 25.0