Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Components of net periodic (benefit) expense
Net actuarial loss recognized in accumulated other comprehensive income	$ (214)	$ (38)	$ (290)	$ (115)
Change in net actuarial loss, tax	(69)	(12)	(93)	(37)
Combined Pension Plan
Components of net periodic (benefit) expense
Service cost	14	15	42	44
Interest cost	49	81	148	243
Expected return on plan assets	(138)	(147)	(414)	(445)
Settlements	57	0	57	0
Recognition of prior service credit	(2)	(3)	(7)	(7)
Recognition of actuarial loss	49	50	147	152
Net periodic benefit expense (income)	29	(4)	$ (27)	(13)
Discount rate			2.79%		2.43%
Net actuarial loss recognized in accumulated other comprehensive income			$ (133)
Change in net actuarial loss, tax			43
Post-Retirement Benefit Plans
Components of net periodic (benefit) expense
Service cost	4	4	11	11
Interest cost	12	17	35	57
Recognition of prior service credit	3	4	11	12
Recognition of actuarial loss	2	0	4	0
Curtailment loss	0	7	0	7
Net periodic benefit expense (income)	$ 21	$ 32	$ 61	$ 87