Components of Long-Term Debt (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Long-term debt		$ 5,909	$ 5,474
Unamortized debt premium, net	[1]	23	31
Unamortized debt issuance costs		(7)	(6)
Interest rate swap fair value adjustment		3	4
Total long-term debt		$ 5,928	5,503
Senior notes | American Water Capital Corp. | Fixed rate
Debt Instrument [Line Items]
Interest Rate, Minimum		3.40%
Interest Rate, Maximum		4.30%
Senior notes | American Water Capital Corp. | Fixed rate | Long Term Debt
Debt Instrument [Line Items]
Interest Rate, Minimum	[2]	3.40%
Interest Rate, Maximum	[2]	8.27%
Weighted Average Rate	[2]	5.23%
Maturity Date, Minimum	[2]	2016
Maturity Date, Maximum	[2]	2045
Long-term debt	[2]	$ 4,273	3,753
Private activity bonds and government funded debt | American Water Capital Corp. | Fixed rate | Long Term Debt
Debt Instrument [Line Items]
Interest Rate, Minimum	[2]	1.79%
Interest Rate, Maximum	[2]	6.25%
Weighted Average Rate	[2]	5.41%
Maturity Date, Minimum	[2]	2021
Maturity Date, Maximum	[2]	2040
Long-term debt	[2]	$ 195	231
Private activity bonds and government funded debt | Other American Water subsidiaries | Fixed rate
Debt Instrument [Line Items]
Interest Rate, Minimum		0.00%
Interest Rate, Maximum		5.00%
Private activity bonds and government funded debt | Other American Water subsidiaries | Fixed rate | Long Term Debt
Debt Instrument [Line Items]
Interest Rate, Minimum	[3]	0.00%
Interest Rate, Maximum	[3]	6.20%
Weighted Average Rate	[3]	4.70%
Maturity Date, Minimum	[3]	2016
Maturity Date, Maximum	[3]	2041
Long-term debt	[3]	$ 790	795
Mortgage bonds | Other American Water subsidiaries | Fixed rate | Long Term Debt
Debt Instrument [Line Items]
Interest Rate, Minimum		4.29%
Interest Rate, Maximum		9.71%
Weighted Average Rate		7.41%
Maturity Date, Minimum		2017
Maturity Date, Maximum		2039
Long-term debt		$ 637	677
Capital lease obligations | Other American Water subsidiaries | Long Term Debt
Debt Instrument [Line Items]
Interest Rate, Minimum		12.23%
Weighted Average Rate		12.23%
Maturity Date, Maximum		2026
Long-term debt		$ 1	1
Mandatorily redeemable preferred stock | Other American Water subsidiaries | Long Term Debt
Debt Instrument [Line Items]
Interest Rate, Minimum		8.47%
Interest Rate, Maximum		9.75%
Weighted Average Rate		8.61%
Maturity Date, Minimum		2019
Maturity Date, Maximum		2036
Long-term debt		$ 13	$ 17

[1] Primarily fair value adjustments previously recognized in acquisition purchase accounting

[2] This indebtedness is considered “debt” for purposes of a support agreement between American Water and American Water Capital Corp. (“AWCC”), which serves as a functional equivalent of a guarantee by AWCC’s payment obligations under such indebtedness.

[3] Includes $8 and $10 of variable rate debt with variable-to-fixed interest rate swaps ranging between 4.65% and 3.93%, as of December 31, 2015 and 2014, respectively. This debt was assumed via an acquisition in 2013