Commitments - Pipeline Charges, Storage Capacity, and Gas Supply Narrative (Details) - Nicor Gas and Southstar MMBTU in Millions, $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) MMBTU
Recorded Unconditional Purchase Obligation [Line Items]
Natural gas pipeline capacity \| MMBTU	47
Long-term purchase commitment amount \| $	$ 150

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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