Retirement Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Retirement Benefits [Abstract]
Service cost	$ 2	$ 2	$ 4	$ 4
Interest cost	3	3	6	7
Expected return on plan assets	(3)	(4)	(8)	(9)
Settlement (gain) loss	0	(6)	0	(6)
Net periodic benefit cost	$ 2	$ (5)	$ 2	$ (4)