Retirement Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Retirement Benefits [Abstract]
Service cost	$ 2	$ 2	$ 4	$ 4
Interest cost	2	3	4	6
Expected return on plan assets	(4)	(5)	(8)	(10)
Net periodic benefit cost	$ 0	$ 0	$ 0	$ 0