Restructuring Costs (Details) $ in Millions	12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($) employees	Sep. 30, 2016 USD ($) employees	Sep. 30, 2015 USD ($) employees
Restructuring Costs [Roll Forward]
Restructuring liability, beginning balance	$ 65	$ 54	$ 70
Restructuring costs		78	96	$ 138
Restructuring, paid/utilized		67	112
Restructuring liability, ending balance		$ 65	$ 54	$ 70
Total number of facilities exited		10	19	12
Total number of positions eliminated | employees		1,200	1,900	3,100
Automation Solutions [Member]
Restructuring Costs [Roll Forward]
Restructuring costs		$ 63	$ 80	$ 102
Commercial & Residential Solutions [Member]
Restructuring Costs [Roll Forward]
Restructuring costs		12	7	31
Climate Technologies [Member]
Restructuring Costs [Roll Forward]
Restructuring costs		10	5	20
Tools & Home Products [Member]
Restructuring Costs [Roll Forward]
Restructuring costs		2	2	11
Severance And Benefits [Member]
Restructuring Costs [Roll Forward]
Restructuring liability, beginning balance	60	44	64
Restructuring costs		49	66
Restructuring, paid/utilized		33	86
Restructuring liability, ending balance		60	44	64
Lease And Other Contract Terminations [Member]
Restructuring Costs [Roll Forward]
Restructuring liability, beginning balance	4	5	1
Restructuring costs		4	9
Restructuring, paid/utilized		5	5
Restructuring liability, ending balance		4	5	1
Asset Write-Downs [Member]
Restructuring Costs [Roll Forward]
Restructuring liability, beginning balance	0	0	0
Restructuring costs		7	4
Restructuring, paid/utilized		7	4
Restructuring liability, ending balance		0	0	0
Vacant Facility And Other Shutdown Costs [Member]
Restructuring Costs [Roll Forward]
Restructuring liability, beginning balance	1	3	3
Restructuring costs		5	7
Restructuring, paid/utilized		7	7
Restructuring liability, ending balance		1	3	3
Start-Up And Moving Costs [Member]
Restructuring Costs [Roll Forward]
Restructuring liability, beginning balance	0	2	2
Restructuring costs		13	10
Restructuring, paid/utilized		15	10
Restructuring liability, ending balance		0	2	2
Scenario, Forecast [Member]
Restructuring Costs [Roll Forward]
Restructuring costs	$ 80
Corporate, Non-Segment [Member] | Corporate and other [Member]
Restructuring Costs [Roll Forward]
Restructuring costs		3	$ 9	$ 5
Pentair's Valves & Controls [Member]
Restructuring Costs [Roll Forward]
Restructuring costs		$ 25