Acquisitions - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) aquisitions	Dec. 31, 2016 USD ($) aquisitions	Dec. 31, 2015 USD ($) aquisitions
Business Acquisition And Divestitures [Line Items]
Number of companies acquired | aquisitions	3
Debt related to purchase price allocation	$ 7
Goodwill	1,379	$ 1,345	$ 1,302
Measurement period adjustments	$ 5
Keystone Clearwater Solutions
Business Acquisition And Divestitures [Line Items]
Purchase price			133
Purchase price allocation, assets acquired			56
Purchase price allocation, liabilities assumed			7
Goodwill			91
Acquisition of nonutility property			25
Account receivable and unbilled revenues			18
Intangible assets			12
Estimated fair value of the redeemable noncontrolling interest			$ 7
Regulated Water And Wastewater Systems
Business Acquisition And Divestitures [Line Items]
Number of companies acquired | aquisitions	18	15	14
Purchase price	$ 210	$ 199	$ 64
Purchase price allocation, assets acquired	207	194	90
Purchase price allocation, liabilities assumed	23	30	26
Contributions in aid of construction	9	14	10
Debt related to purchase price allocation	7	$ 6	1
Goodwill			3
Bargain purchase gain recognized	$ 3		3
Business combination, purchase gain deferred as regulatory liability			$ 1
Regulated Businesses | Regulated Water And Wastewater Systems
Business Acquisition And Divestitures [Line Items]
Number of companies acquired | aquisitions	4	5
Goodwill	$ 29	$ 43
Goodwill, expected tax deductible amount	1	$ 31
Measurement period adjustments	$ 5