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FAIR VALUE MEASUREMENTS - UNREALIZED GAIN (LOSS) ON INVESTMENTS (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Fair Value Disclosures [Abstract]        
Net unrealized gains included in earnings related to investments held in rabbi trust $ 7.1 $ 4.4 $ 7.5 $ 12.2