NET INCOME PER SHARE (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic
Net income	$ 595.9	$ 321.7	$ 471.0	$ 245.2	$ 917.6	$ 716.2
Average shares outstanding (in shares)	90,469,663		91,775,295		90,772,818	91,864,062
Basic net income per share (in dollars per share)	$ 6.59		$ 5.13		$ 10.11	$ 7.80
Diluted
Net income	$ 595.9	$ 321.7	$ 471.0	$ 245.2	$ 917.6	$ 716.2
Average shares outstanding assuming dilution:
Average shares outstanding (in shares)	90,469,663		91,775,295		90,772,818	91,864,062
Stock options and other contingently issuable shares (in shares)	1,463,195		1,733,040		1,644,840	1,649,069
Non-vested restricted stock grants (in shares)	19,940		53,390		20,117	53,496
Average shares outstanding assuming dilution (in shares)	91,952,798		93,561,725		92,437,775	93,566,627
Diluted net income per share (in dollars per share)	$ 6.48		$ 5.03		$ 9.93	$ 7.65
Stock options and other contingently issuable shares with anti-dilutive effects (in shares)	469,235		0		469,235	17,286