SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recently Issued and Adopted Accounting Standards (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Liabilities			$ 314.3
Operating Lease, Right-of-Use Asset			264.3
Liabilities			24,906.5	$ 28,055.6
Accrued expenses			327.8	322.2
Other liabilities			$ 337.7	$ 392.4
ASC 842
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Liabilities	$ 324.5
Operating Lease, Right-of-Use Asset	269.9
Cumulative effect on retained earnings	0.0
Discontinued Operations, Held-for-sale | Epsilon | ASC 842
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Liabilities	208.7
Operating Lease, Right-of-Use Asset	181.1
Capital lease assets	13.0
Capital lease liabilities	12.6
Forecast | ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for loan and lease losses, adjustments, net		$ 644.0
Adjustment | ASC 842
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Right-of-Use Asset	54.6
Liabilities	(54.6)
Adjustment | Discontinued Operations, Held-for-sale | Epsilon | ASC 842
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Right-of-Use Asset	2.9
Liabilities	$ 30.5