Pension and Other Postretirement Benefits - Summary of Information Regarding Net Periodic Benefit Cost Included in Statement of Income Related to Significant Defined Benefit Pension and Other Postretirement Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension
Components of net periodic benefit cost:
Service cost	$ 52	$ 60	$ 63
Interest cost	78	72	72
Expected return on plan assets	(121)	(126)	(133)
Amortization of actuarial (gain) loss	21	43	57
Amortization of prior service cost	1	0	0
Total net periodic benefit cost	31	49	59
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	7	8	9
Interest cost	20	18	19
Expected return on plan assets	(22)	(25)	(23)
Amortization of actuarial (gain) loss	(1)	(2)	(1)
Amortization of prior service cost	0	0	0
Total net periodic benefit cost	$ 4	$ (1)	$ 4