Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7	$ 7
Interest cost	47	59
Expected return on plan assets	(70)	(72)
Recognized net actuarial loss	42	35
Settlements	0	2
Net periodic benefit cost	26	31
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	2
Interest cost	6	7
Recognized net actuarial loss	2	1
Amortization of prior service cost (benefit)	(2)	(1)
Curtailments	0	(58)
Net periodic benefit cost	$ 7	$ (49)