Variable Interest Entities - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jun. 30, 2000	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Variable Interest Entity [Line Items]
Aggregate investment balance		$ 173	$ 186
Net income (loss) attributable to noncontrolling interests		(2)	(1)	$ 40
Trust Funds for Final Capping Closure Post Closure or Environmental Remediation Obligations [Member]
Variable Interest Entity [Line Items]
Carrying value of trusts for which company is not the sole beneficiary		93	93
Variable interest entities primary beneficiary trust fair value assets		95	94
Investment in Refined Coal Facility and Low Income Housing Properties [Member]
Variable Interest Entity [Line Items]
Aggregate investment balance		84	110
Investment in Low-Income Housing Properties [Member]
Variable Interest Entity [Line Items]
Equity method investments debt balance		$ 57	$ 80
Waste To Energy LLC [Member]
Variable Interest Entity [Line Items]
Net income (loss) attributable to noncontrolling interests				$ 39
Waste To Energy LLC [Member] | Waste To Energy LLC 1 [Member]
Variable Interest Entity [Line Items]
Percentage of ownership in LLC's	0.50%
Percentage of variable interest entities owned by other companies	99.50%
Waste To Energy LLC [Member] | Waste To Energy LLC II [Member]
Variable Interest Entity [Line Items]
Percentage of ownership in LLC's	0.25%
Percentage of variable interest entities owned by other companies	99.75%