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Other Long-term Liabilities - Components of Deferred Revenue (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Deferred Revenue Arrangement [Line Items]      
Total deferred revenue $ 168.5 $ 172.0 $ 175.5
Splitter Agreement [Member]      
Deferred Revenue Arrangement [Line Items]      
Total deferred revenue 129.0 129.0  
Gas Contract Amendment [Member]      
Deferred Revenue Arrangement [Line Items]      
Total deferred revenue 37.3 39.8  
Other Deferred Revenue [Member]      
Deferred Revenue Arrangement [Line Items]      
Total deferred revenue $ 2.2 $ 3.2