Investment in Transmission Affiliates - ATC Summarized Financial Data (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Investment in transmission affiliates
Operating revenues	$ 8,316.0	$ 7,241.7	$ 7,523.1
Operating expenses	6,601.1	5,535.6	5,991.7
Other expense, net	216.1	276.8	271.7
Assets, Current	2,656.7	2,083.0
Noncurrent assets	36,331.8	34,945.1
Total assets	38,988.5	37,028.1	34,951.8
Liabilities, Current	3,753.0	4,148.1
Long-term debt	13,472.4	11,672.8
Other noncurrent liabilities	1,203.2	1,229.4
Total liabilities and members' equity	38,988.5	37,028.1
ATC
Investment in transmission affiliates
Operating revenues	754.8	758.1	744.4
Operating expenses	376.2	372.5	373.5
Other expense, net	113.9	110.8	110.5
Net income	264.7	274.8	$ 260.4
Assets, Current	89.8	92.7
Noncurrent assets	5,628.1	5,400.6
Total assets	5,717.9	5,493.3
Liabilities, Current	436.9	310.8
Long-term debt	2,513.0	2,512.2
Other noncurrent liabilities	422.0	378.2
Members' equity	2,346.0	2,292.1
Total liabilities and members' equity	$ 5,717.9	$ 5,493.3