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STOCK-BASED COMPENSATION. (Stock Options) (Details)
3 Months Ended
Mar. 31, 2021
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]  
Outstanding, beginning balance (in shares) | shares 4,329,056
Exercised (in shares) | shares (748,203)
Outstanding, ending balance (in shares) | shares 3,580,853
Exercisable at current period-end (in shares) | shares 3,580,853
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]  
Weighted-average exercise price of options outstanding, beginning balance (in dollars per share) | $ / shares $ 72.52
Weighted-average exercise price of options exercised (in dollars per share) | $ / shares 72.28
Weighted-average exercise price of options outstanding, ending balance (in dollars per share) | $ / shares 72.57
Weighted-average exercise price of options exercisable at current period-end (in dollars per share) | $ / shares $ 72.57