Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Sep. 30, 2019
Fair Value of Financial Instruments [Line Items]
Investment excludes receivables, payables and accrued income	$ 4	$ (15)
Cash balance excluded from available for sale securities	20	33
Investments not required to be disclosed	253	283
Nuclear plant decommissioning trusts	$ 790	871
NUG contracts
Fair Value of Financial Instruments [Line Items]
Period of future observable data to determine contract price	2 years
Nuclear Plant Matters \| JCP&L, ME and PN \| Financial Guarantee
Fair Value of Financial Instruments [Line Items]
Nuclear plant decommissioning trusts		$ 900

Decommission fund to pay for the costs of decontaminating and decommissioning of facilities through collection of revenues derived from utility assessments and government appropriations. Decommission fund investment for the process whereby a power station, at the end of its economic life, is taken permanently out of service and its site made available for other purposes. In the case of a nuclear station this comprises three different states of clearance. Immediately after the final closure, radioactive material such as nuclear fuel and operational waste is removed and the buildings surrounding the reactor shield are dismantled and finally the reactor itself is dismantled.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_DecommissioningFundInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant