CONDENSED BALANCE SHEETS - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Current assets
Cash and cash equivalents	$ 1.9	$ 19.1
Accounts receivable and unbilled revenues, net of reserves of $52.1 and $38.1, respectively	433.0	434.6
Accounts receivable from related parties	78.8	86.5
Materials, supplies, and inventories	230.1	229.8
Prepaid taxes	69.9	104.4
Other	20.9	33.6
Current assets	834.6	908.0
Assets, Noncurrent [Abstract]
Property, plant, and equipment, net of accumulated depreciation and amortization of $4,806.3 and $4,564.0, respectively	9,726.5	9,586.7
Regulatory assets	2,751.0	2,755.2
Other	111.4	110.9
Long-term assets	12,588.9	12,452.8
Total assets	13,423.5	13,360.8
Current liabilities
Short-term debt	0.0	115.5
Current portion of long-term debt	300.0	0.0
Current portion of finance lease obligations	64.6	57.8
Accounts payable	239.6	267.6
Accounts payable to related parties	144.9	184.5
Accrued payroll and benefits	44.7	51.3
Other	145.2	117.9
Current liabilities	939.0	794.6
Liabilities, Noncurrent [Abstract]
Long-term debt	2,460.6	2,759.2
Finance lease obligations	2,789.8	2,783.1
Deferred income taxes	1,361.3	1,347.4
Regulatory liabilities	1,707.2	1,744.2
Pension and OPEB obligations	45.0	59.8
Other	274.6	281.0
Long-term liabilities	8,638.5	8,974.7
Commitments and contingencies (Note 17)
Equity [Abstract]
Common stock – $10 par value; 65,000,000 shares authorized; 33,289,327 shares outstanding	332.9	332.9
Additional paid in capital	1,060.1	929.5
Retained earnings	2,422.6	2,298.7
Common shareholder's equity	3,815.6	3,561.1
Preferred stock	30.4	30.4
Total liabilities and equity	$ 13,423.5	$ 13,360.8