SHARE INCENTIVE PLANS - Summary of share option activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number of Shares
Options outstanding at the beginning of the period (in shares)	494,503	1,110,061
Options exercised (in shares)	(219,458)	(586,076)
Options canceled (in shares)	(1,040)	(29,482)
Options outstanding at the end of the period (in shares)	274,005	494,503
Weighted Average Exercise Price
Options outstanding at the beginning of the period (in USD per share)	$ 50.73	$ 38.35
Options exercised (in USD per share)	34.63	24.77
Options canceled (in USD per share)	91.91	100.81
Options outstanding at the end of the period (in USD per share)	$ 63.46	$ 50.73
Weighted average remaining contractual life	2 years 7 months 6 days
Share options
Weighted Average Exercise Price
Total pretax intrinsic value of options exercised	$ 15.6	$ 33.5
Share value (in USD per share)	$ 117.18	$ 92.38
Total pretax intrinsic value of all outstanding options	$ 14.7	$ 21.8
Total pretax intrinsic value of all exercisable options	$ 14.7	$ 21.7
Number of exercisable options outstanding (in shares)	273,963
Weighted average exercise price, options exercisable (in USD per share)	$ 63.47