SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - RECENTLY ADOPTED ACCOUNTING PRONOUNCEMENT BALANCE SHEET (Details) - USD ($) $ in Millions	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Jan. 01, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax assets	$ 984.6		$ 1,029.8			$ 1,045.9
Valuation allowance for deferred income tax assets	0.0		(193.7)		$ (89.1)	(255.9)	$ (240.2)
Net deferred tax assets	428.9		604.7			790.0
Total assets	33,630.9	$ 31,436.7	31,439.8			31,975.5
Additional paid-in capital	2,767.3		2,995.0			3,213.0
Accumulated other comprehensive income	1,372.5		177.7	$ 1,195.8
Retained earnings	535.7	193.5	196.6	576.7		650.1
Total shareholders' equity	4,677.0	$ 3,367.8	3,370.9	4,847.5	4,847.5	4,487.2	4,486.9
Total liabilities and shareholders' equity	$ 33,630.9		31,439.8			31,975.5
Accounting Standards Update 2016-09, Election to Account for Forfeitures as they Occur
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax assets						0.3
Valuation allowance for deferred income tax assets						0.0
Net deferred tax assets						0.3
Total assets						0.3
Additional paid-in capital						0.9
Retained earnings						(0.6)
Total shareholders' equity						0.3
Total liabilities and shareholders' equity						0.3
Accounting Standards Update 2016-09, Recognition of Excess Tax Benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax assets					$ 15.7	15.7
Valuation allowance for deferred income tax assets						(15.7)
Net deferred tax assets						0.0
Total assets						0.0
Additional paid-in capital						0.0
Retained earnings						0.0
Total shareholders' equity						0.0
Total liabilities and shareholders' equity						$ 0.0
Amounts prior to effect of adoption of authoritative guidance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax assets							1,029.9
Valuation allowance for deferred income tax assets							(240.2)
Net deferred tax assets							789.7
Total assets			31,439.8				31,975.2
Additional paid-in capital							3,212.1
Accumulated other comprehensive income				1,212.1
Retained earnings			196.6	560.4			650.7
Total shareholders' equity			$ 3,370.9	4,847.5			4,486.9
Total liabilities and shareholders' equity							$ 31,975.2
Restatement adjustment | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated other comprehensive income				(16.3)
Retained earnings				16.3
Total shareholders' equity				$ 0.0