Investment in Transmission Affiliates - ATC Summarized Financial Data (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Investment in transmission affiliates
Operating revenues	$ 9,597.4	$ 8,316.0	$ 7,241.7
Operating expenses	7,673.2	6,601.1	5,535.6
Other expense, net	191.6	216.1	276.8
Current assets	3,187.7	2,656.7
Noncurrent assets	38,684.4	36,331.8
Total assets	41,872.1	38,988.5	37,028.1
Current liabilities	4,611.0	3,753.0
Long-term debt	14,655.7	13,472.4
Other noncurrent liabilities	1,475.3	1,203.2
Total liabilities and members' equity	41,872.1	38,988.5
ATC
Investment in transmission affiliates
Operating revenues	751.2	754.8	758.1
Operating expenses	381.5	376.2	372.5
Other expense, net	123.0	113.9	110.8
Net income	246.7	264.7	$ 274.8
Current assets	89.6	89.8
Noncurrent assets	5,997.8	5,628.1
Total assets	6,087.4	5,717.9
Current liabilities	511.9	436.9
Long-term debt	2,613.0	2,513.0
Other noncurrent liabilities	485.8	422.0
Members' equity	2,476.7	2,346.0
Total liabilities and members' equity	$ 6,087.4	$ 5,717.9