Regulatory Assets and Liabilities - Regulatory Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Regulatory liabilities
Current liabilities	$ 12.0	$ 11.9
Regulatory liabilities	1,744.2	2,002.3
Total regulatory liabilities	1,756.2	2,014.2
Income tax related items
Regulatory liabilities
Total regulatory liabilities	888.1	1,024.8
Removal costs
Regulatory liabilities
Total regulatory liabilities	654.7	748.1
Pension and OPEB costs
Regulatory liabilities
Total regulatory liabilities	120.4	74.7
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	38.6	0.0
Refunds due to customers offset against electric transmission	38.6
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	28.8	16.4
Energy efficiency programs
Regulatory liabilities
Total regulatory liabilities	15.8	13.5
Mines deferral
Regulatory liabilities
Total regulatory liabilities	0.0	120.8
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 9.8	$ 15.9