Regulatory Assets and Liabilities - Regulatory Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Regulatory liabilities
Current liabilities	$ 36.8	$ 41.8
Regulatory liabilities	4,251.6	3,718.6
Total regulatory liabilities	4,288.4	3,760.4
Income tax related items
Regulatory liabilities
Total regulatory liabilities	2,406.6	2,134.1
Removal costs
Regulatory liabilities
Total regulatory liabilities	1,329.6	1,294.9
Pension and OPEB costs
Regulatory liabilities
Total regulatory liabilities	238.3	114.2
Mines deferral
Regulatory liabilities
Total regulatory liabilities	120.8	95.1
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	39.6	42.0
Energy efficiency programs
Regulatory liabilities
Total regulatory liabilities	31.7	21.1
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	30.5	24.7
Decoupling
Regulatory liabilities
Total regulatory liabilities	30.5	1.8
Earnings sharing mechanisms
Regulatory liabilities
Total regulatory liabilities	30.0	2.5
Derivatives
Regulatory liabilities
Total regulatory liabilities	16.4	11.0
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 14.4	$ 19.0