REVENUE RECOGNITION - Rollforward of contract liabilities (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Change in Contract with Customer, Asset and Liability [Abstract]
Beginning balance	$ 15.4
Cash Additions	12.8
Revenue Recognition	(10.8)
Ending balance	17.4
Liquidity provider sliding scale
Change in Contract with Customer, Asset and Liability [Abstract]
Beginning balance	4.8
Cash Additions	4.8
Revenue Recognition	(4.8)
Ending balance	4.8
Other, net
Change in Contract with Customer, Asset and Liability [Abstract]
Beginning balance	10.6
Cash Additions	8.0
Revenue Recognition	(6.0)
Ending balance	$ 12.6