INCOME TAXES - Reconciliation of Federal Statutory Income Tax Rate to Effective Income Tax Rate (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax expense at U.S. federal statutory rate			$ 245.0	$ 159.9	$ 362.2
Foreign income taxed at different rates			(75.0)	(16.2)	(106.9)
State and local taxes, net of federal tax benefit			11.9	6.8	11.5
Tax credits, federal and state			(42.9)	(36.7)	(25.8)
Build (release) of reserve for prior years' uncertain tax positions			5.0	(35.5)	(7.7)
U.S. tax on foreign earnings, net of credits			(2.9)	(12.2)	(30.3)
Tax on global intangible low-taxed income	$ 0.0	$ 0.0	32.0
Foreign-derived intangible income deduction			(7.2)	(6.6)	0.0
Deductible employee share-based compensation			(57.6)	(41.8)	(48.2)
Nondeductible employee share-based compensation			3.2	2.8	3.9
Impact related to 2017 U.S. Tax Reform			2.8	15.8	294.1
Other			5.3	2.9	(1.5)
Total income tax provision			$ 119.6	$ 39.2	$ 451.3