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Revenue from Contracts with Customers (Tables)
 12 Months Ended
Dec. 31, 2018
Revenue from Contract with Customer [Abstract]  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction The following table includes estimated revenues based upon those certain agreements with fixed minimum take-or-pay sales volumes. Our actual future sales volumes under these agreements may exceed future minimum volume commitments.
(millions)
2019
2020
Total
Natural Gas Revenues (1)
$
137

$
169

$
306

(1) 
The remaining performance obligations are estimated utilizing the contractual base or floor price provision in effect. Our future revenues from the sale of natural gas under these associated contracts will vary from the amounts presented above due to components of variable consideration above the contractual base or floor provision, such as index-based escalations and market price changes.