Acquisitions and Divestitures Narrative (Details) $ / shares in Units, $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Feb. 10, 2018 USD ($)	Dec. 01, 2017 USD ($)	Apr. 28, 2017 USD ($)
Business Acquisition & Disposal [Line Items]
Net sales	$ 3,816	$ 3,216
Goodwill	5,616			$ 5,616	$ 5,316
Purchase price of businesses, net of cash acquired	513	16
Proceeds from divestiture of businesses	235	0
Pretax earnings	503	464
Net income (loss)	$ 394	$ 370
Diluted earnings per common share | $ / shares	$ 0.61	$ 0.48
Paradigm [Member]
Business Acquisition & Disposal [Line Items]
Cash paid, net of cash acquired							$ 505
Net sales				140
Goodwill							304
Goodwill expected tax deductible amount							160
Intangibles							$ 248
Intangible assets, weighted average useful life	11 years
Pentair's Valves & Controls [Member]
Business Acquisition & Disposal [Line Items]
Cash paid, net of cash acquired								$ 2,960
Net sales					1,400
Cash acquired								$ 207
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | ClosetMaid [Member]
Business Acquisition & Disposal [Line Items]
Proceeds from divestiture of businesses	$ 200
Gain (loss) on divestiture of business, after tax	$ (24)
Disposal gain (loss) impact on diluted earnings per share | $ / shares	$ (0.04)
Current assets held-for-sale					73
Other noncurrent assets held-for-sale					176
Accrued expenses & other liabilities held-for-sale					61
Scenario, Forecast [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | ClosetMaid [Member]
Business Acquisition & Disposal [Line Items]
After-tax proceeds on sale of business			$ 150
Fair Value Adjustment To Inventory & Backlog [Member] | Pentair's Valves & Controls [Member]
Business Acquisition & Disposal [Line Items]
Pretax earnings	$ (25)
Net income (loss)	$ (19)
Diluted earnings per common share | $ / shares	$ (0.03)
Automation Solutions [Member]
Business Acquisition & Disposal [Line Items]
Goodwill	$ 5,005			$ 5,005	$ 4,704
Automation Solutions [Member] | Series of Individually Immaterial Business Acquisitions [Member]
Business Acquisition & Disposal [Line Items]
Number of additional businesses acquired	1
Subsequent Event [Member] | Cooper-Atkins [Member]
Business Acquisition & Disposal [Line Items]
Cash paid, net of cash acquired						$ 247