REVENUES - Disaggregation of Revenues (Details) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2018 USD ($) customer	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)
Disaggregation of Revenues
Provision for revenue sharing			$ 10.0
Contract balances	$ 44.0	$ 44.0	44.0	$ 40.0
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