STOCK-BASED COMPENSATION - Equity Awards (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Stock options
Stock-based compensation
Weighted-average grant-date fair value of options granted (in dollars per share)	$ 22.82	$ 18.00
Total intrinsic value of options exercised	$ 18.2	$ 32.4
Cash received from option exercises	6.5	$ 13.6
Total unrecognized compensation expense related to non-vested shares granted to employees	$ 125.3
Cost expected to be recognized over a weighted-average period	3 years 2 months 12 days
Stock units expected to vest (in shares)	18,689
Restricted stock units
Stock-based compensation
Total unrecognized compensation expense related to non-vested shares granted to employees	$ 23.1
Cost expected to be recognized over a weighted-average period	2 years 1 month 6 days
Stock units expected to vest (in shares)	500
Number of Shares
Non-vested at the beginning of the period (in shares)	530
Granted (in shares)	221
Vested (in shares)	(255)
Forfeited/cancelled (in shares)	(6)
Non-vested at the end of the period (in shares)	490
Weighted Average Grant-Date Fair Value
Non-vested at the beginning of the period (in dollars per share)	$ 45.09
Granted (in dollars per share)	58.75	$ 46.27
Vested (in dollars per share)	45.62
Forfeited/cancelled (in dollars per share)	32.17
Non-vested at the end of the period (in dollars per share)	$ 51.15