Retirement Benefits Retirement Benefits - Gross Accumulated Other Comprehensive Loss and Regulatory Assets (Liabilities) not Recognized as Components of Periodic Benefit Cost (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	[1]
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service (credit) cost	$ (24)	$ (31)
Actuarial loss	7,556	8,387
Total	7,532	8,356
Benefits included in accumulated other comprehensive income	3,896	4,297
Benefits included in regulatory assets	3,636	4,059
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service (credit) cost	(522)	(710)
Actuarial loss	829	724
Total	307	14
Benefits included in accumulated other comprehensive income	125	(42)
Benefits included in regulatory assets	$ 182	$ 56

[1] Effective March 23, 2016, Exelon became the sponsor of PHI's defined benefit pension and other postretirement benefit plans, and assumed PHI's benefit plan obligations and related assets.