Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7	$ 20
Interest cost	59	55
Expected return on plan assets	(72)	(77)
Recognized net actuarial loss	35	42
Amortization of prior service cost (benefit)	0	1
Settlements	2		$ 0
Curtailments	0	5
Net periodic benefit cost	31	46
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	2
Interest cost	7	7
Recognized net actuarial loss	1	2
Amortization of prior service cost (benefit)	(1)	(2)
Curtailments	(58)	0
Net periodic benefit cost	$ (49)	$ 9