OTHER ASSETS, NET - Schedule of other assets, net (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Finite-Lived Intangible Assets
Less accumulated depreciation and amortization	$ (187.1)		$ (187.1)		$ (189.6)
Data processing software, net	37.5		37.5		40.6
Other assets	22.1		22.1		18.9
Other assets, net	59.6		59.6		59.5
Amortization	39.7	$ 42.7	81.8	$ 57.2
Software development work in progress
Finite-Lived Intangible Assets
Software	7.1		7.1		10.2
Data processing software
Finite-Lived Intangible Assets
Software	217.5		217.5		$ 220.0
Amortization	$ 4.6	$ 4.4	$ 9.7	$ 8.5