Statements of Changes in Shareholder's Equity (Unaudited) - MEC - USD ($) $ in Millions	Total	Additional Paid-in Capital	MEC	MEC Common Stock	MEC Additional Paid-in Capital	MEC Retained Earnings
Balance at Dec. 31, 2019			$ 7,240	$ 0	$ 561	$ 6,679
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 4,628		700			700
Other equity transactions	0	$ (1)
Balance at Sep. 30, 2020			7,940	0	561	7,379
Balance at Jun. 30, 2020			7,600	0	561	7,039
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	2,842		340			340
Other equity transactions	1
Balance at Sep. 30, 2020			7,940	0	561	7,379
Balance at Dec. 31, 2020	43,010		8,065	0	561	7,504
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	4,207		737			737
Other equity transactions	(3)
Balance at Sep. 30, 2021	45,692		8,802	0	561	8,241
Balance at Jun. 30, 2021			8,426	0	561	7,865
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	1,922		377			377
Other equity transactions	(2)		(1)			(1)
Balance at Sep. 30, 2021	$ 45,692		$ 8,802	$ 0	$ 561	$ 8,241