Retirement Benefits Retirement Benefits - Balance Sheet locations of Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	[1]
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	$ 28	$ 21
Pension obligations	3,736	4,248
Non-pension postretirement benefit obligations	0	0
Unfunded status (net benefit obligation less plan assets)	3,764	4,269
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	31	31
Pension obligations	0	0
Non-pension postretirement benefit obligations	2,093	1,848
Unfunded status (net benefit obligation less plan assets)	$ 2,124	$ 1,879

[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.