Contingencies - GAS Litigation (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($)
Schedule of Regulatory Assets and Liabilities [Line Items]
Minimum insurance coverage	$ 11
Nicor Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Litigation settlement, expense	$ 11

Amount of litigation expense, including but not limited to legal, forensic, accounting, and investigative fees.

No definition available.

Name:	us-gaap_LitigationSettlementExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration