REVENUES - Disaggregation of Revenues (Details) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2018 USD ($) customer	Dec. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Disaggregation of Revenues
Provision For revenue sharing		$ 10.0
Contract balances			$ 36.0	$ 37.0
Bison
Disaggregation of Revenues
Non-refundable receipt from contract termination		$ 97.0
Number of customers that terminated transportation agreement | customer	2
Bison | Tenaska
Disaggregation of Revenues
Non-refundable receipt from contract termination	$ 95.4
Bison | Another customer
Disaggregation of Revenues
Non-refundable receipt from contract termination	$ 2.0