Acquisitions and Divestitures Pro Forma Financial Information (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Business Acquisition [Line Items]
Net income (loss)	$ 1,990	$ 2,328	$ 2,224
Earnings before income taxes	$ 2,335	$ 2,859	2,667
Net sales			18,200
Net earnings from continuing operations common stockholders			$ 2,300
Diluted earnings per share from continuing operations			$ 3.56
Current year acquired businesses and Pentair's Valves & Controls [Member] | Acquisition-related costs and first year acquisition accounting charges [Member] [Member]
Business Acquisition [Line Items]
Earnings before income taxes			$ (102)
Total acquired businesses
Business Acquisition [Line Items]
Net income (loss)			(3)
Total acquired businesses | Acquisition-related costs and first year acquisition accounting charges [Member] [Member]
Business Acquisition [Line Items]
Earnings before income taxes			(73)
Pentair's Valves & Controls [Member] | First year acquisition accounting charges [Member]
Business Acquisition [Line Items]
Earnings before income taxes			$ (29)