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SIGNIFICANT ACCOUNTING POLICIES - Property, Plant and Equipment (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Property, Plant and Equipment      
Depreciation expenses $ 88.0 $ 78.0 $ 97.0
AFUDC - Equity 10.0 2.0 1.0
AFUDC debt $ 1.3 $ 1.0 $ 1.0
Pipeline facilities and compression equipment | Minimum      
Property, Plant and Equipment      
Estimated useful lives 20 years    
Pipeline facilities and compression equipment | Maximum      
Property, Plant and Equipment      
Estimated useful lives 68 years    
Metering and other | Minimum      
Property, Plant and Equipment      
Estimated useful lives 5 years    
Metering and other | Maximum      
Property, Plant and Equipment      
Estimated useful lives 77 years