STOCKHOLDERS' EQUITY - Stock Repurchase Programs (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	6 Months Ended	12 Months Ended
	Aug. 15, 2019	Jul. 31, 2017	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jul. 31, 2019	Jul. 19, 2019	Jul. 31, 2018	Jul. 26, 2018	Jan. 31, 2017	Jul. 01, 2015	Jul. 01, 2010
Stock Repurchase Programs
Number of shares repurchased				6,300,000	2,200,000	2,300,000
Total cost of shares repurchased				$ 976.1	$ 443.2	$ 553.7
2017 Stock Repurchase Program
Stock Repurchase Programs
Amount of company's outstanding common stock authorized to be repurchased						$ 1,000.0					$ 500.0
Increase to authorized amount under stock repurchase program		$ 500.0
Number of shares repurchased					800,000
Total cost of shares repurchased					$ 166.0
Amount remaining of a stock repurchase plan that expired									$ 280.3
2018 Stock Repurchase Program
Stock Repurchase Programs
Amount of company's outstanding common stock authorized to be repurchased										$ 500.0
Number of shares repurchased			1,300,000		1,400,000
Total cost of shares repurchased			$ 222.8		$ 277.2
2019 Stock Repurchase Program
Stock Repurchase Programs
Amount of company's outstanding common stock authorized to be repurchased							$ 1,100.0
Number of shares repurchased	5,050,505
Average cost per share (in dollars per share)	$ 148.50
Total cost of shares repurchased	$ 750.0
Available balance under stock repurchase program				$ 347.8
Total direct costs	3.3
Commission expense	$ 2.2
Modified Dutch Auction
Stock Repurchase Programs
Amount of company's outstanding common stock authorized to be repurchased								$ 750.0
Modified Dutch Auction | Minimum
Stock Repurchase Programs
Share price (in dollars per share)								$ 144.00
Modified Dutch Auction | Maximum
Stock Repurchase Programs
Share price (in dollars per share)								$ 162.00
2010 Omnibus Incentive Plan
Stock Compensation Plans
Shares of common stock reserved for grant													3,000,000
2015 Omnibus Incentive Plan
Stock Compensation Plans
Shares of common stock reserved for grant												5,100,000