Variable Interest Entities - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended			3 Months Ended		9 Months Ended
	Jan. 31, 2011	Apr. 30, 2010	Jun. 30, 2000	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Variable Interest Entity [Line Items]
Net income (loss) attributable to noncontrolling interests				$ 2	$ 11	$ (1)	$ 32
Trust for Final Capping Closure Post Closure or Environmental Remediation Obligations [Member]
Variable Interest Entity [Line Items]
Fair value of trusts for which company is sole beneficiary				91		91		$ 129
Carrying value of trusts for which company is not the sole beneficiary				99		99		113
Investment in Refined Coal Facility [Member]
Variable Interest Entity [Line Items]
Cash payment to acquire equity method investment	$ 48
Investment balance				30		30		32
Low-Income Housing Properties [Member]
Variable Interest Entity [Line Items]
Cash payment to acquire equity method investment		$ 6
Investment balance				87		87		104
Consideration for investment		221
Consideration for investment, notes payable		$ 215
Equity method investments debt balance				$ 86		$ 86		$ 104
Waste To Energy LLC [Member]
Variable Interest Entity [Line Items]
Net income (loss) attributable to noncontrolling interests					$ 8		$ 29
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%