Pension and Other Postretirement Benefits - Additional Information (Detail) (USD $)	0 Months Ended	12 Months Ended
	Jul. 01, 2013	Dec. 31, 2014	Dec. 31, 2013 Amendments	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation		$ 2,400,000,000	$ 2,100,000,000
Service periods for participating employee		11 years	11 years
Percentage of plan asset target allocation		68.00%
Unfunded capital commitments		11,000,000	12,000,000
Pension contribution			120,000,000	200,000,000
Percentage of participants vested		100.00%
Defined contribution plan matching contributions		13,000,000	12,000,000	12,000,000
Other postretirement benefit obligations		265,000,000	206,000,000
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net loss to be amortized from AOCL or regulatory assets into net periodic benefit cost		63,000,000
Prior service cost to be amortized from AOCL or regulatory assets into net periodic benefit cost		2,000,000
Reduction of the projected benefit obligation			3,000,000
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net loss to be amortized from AOCL or regulatory assets into net periodic benefit cost		11,000,000
Prior service cost to be amortized from AOCL or regulatory assets into net periodic benefit cost		13,000,000
Reduction of the projected benefit obligation			(124,000,000)
OPEB contributions		1,000,000	6,000,000
Other postretirement benefit obligations		265,000,000	206,000,000
Potomac Electric Power Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension contribution		0	0	85,000,000
Pension and other postretirement net periodic benefit cost		22,000,000	34,000,000	39,000,000
Prepaid pension expense		316,000,000	332,000,000
Other postretirement benefit obligations		57,000,000	61,000,000
Potomac Electric Power Co [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of amendments, PHI approved			2
Reduction in net periodic benefit cost		7,000,000
Percentage of postretirement benefit costs		40.00%
OPEB contributions		1,000,000	6,000,000	5,000,000
Delmarva Power & Light Co/De [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension contribution		0	10,000,000	85,000,000
Pension and other postretirement net periodic benefit cost		7,000,000	18,000,000	23,000,000
Prepaid pension expense		220,000,000	228,000,000
Other postretirement benefit obligations		21,000,000	23,000,000
Delmarva Power & Light Co/De [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of amendments, PHI approved			2
Reduction in net periodic benefit cost		4,000,000
OPEB contributions		0	3,000,000	7,000,000
Delmarva Power & Light Co/De [Member] | Retiree [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension contribution		0	10,000,000
PHI Service Company [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of amendments, PHI approved			2
Reduction in net periodic benefit cost			193,000,000	69,000,000
Prior service credit included in projected benefit obligation			124,000,000
Amortization period			10 years
Change in the discount rate	4.95%			4.10%
Reduction of the projected benefit obligation		19,000,000
Percentage of postretirement benefit costs		36.00%
PHI Service Company [Member] | Retiree [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension contribution		0	80,000,000
Atlantic City Electric Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension contribution		0	30,000,000	30,000,000
Pension and other postretirement net periodic benefit cost		13,000,000	17,000,000	24,000,000
Prepaid pension expense		96,000,000	106,000,000
Other postretirement benefit obligations		36,000,000	35,000,000
Atlantic City Electric Co [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of amendments, PHI approved			2
Reduction in net periodic benefit cost		3,000,000
Percentage of postretirement benefit costs		45.00%
OPEB contributions		3,000,000	6,000,000	7,000,000
Atlantic City Electric Co [Member] | Retiree [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension contribution		0	30,000,000
Subsidiaries [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
OPEB contributions		$ 0	$ 7,000,000