Regulatory Assets and Liabilities - Regulatory Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Regulatory liabilities
Current liabilities	$ 87.6	$ 36.8
Regulatory liabilities	3,992.8	4,251.6
Total regulatory liabilities	4,080.4	4,288.4
Income tax related items
Regulatory liabilities
Total regulatory liabilities	2,248.8	2,406.6
Removal costs
Regulatory liabilities
Total regulatory liabilities	1,181.5	1,329.6
Pension and OPEB costs
Regulatory liabilities
Total regulatory liabilities	354.9	238.3
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	89.8	39.6
Earnings sharing mechanisms
Regulatory liabilities
Total regulatory liabilities	43.5	30.0
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	42.2	9.7
Refunds due to customers offset against electric transmission regulatory liability	38.6
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	39.1	30.5
Decoupling
Regulatory liabilities
Total regulatory liabilities	36.8	30.5
Energy efficiency programs
Regulatory liabilities
Total regulatory liabilities	30.7	31.7
Derivatives
Regulatory liabilities
Total regulatory liabilities	6.7	16.4
Mines deferral
Regulatory liabilities
Total regulatory liabilities	0.0	120.8
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 6.4	$ 4.7