Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Retirement Benefits [Abstract]
Service cost	$ 1	$ 1
Interest cost	10	13
Expected return on assets	(26)	(26)
Amortization of prior service credit / actuarial loss	4	4
Net periodic benefit cost	$ (11)	$ (8)