FAIR VALUE MEASUREMENTS - FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Financial instruments
Preferred stock of subsidiary	$ 30.4	$ 30.4
Carrying amount
Financial instruments
Preferred stock of subsidiary	30.4	30.4
Long-term debt, including current portion	13,114.9	12,450.5
Finance lease obligations	59.9	63.4
Fair value
Financial instruments
Preferred stock of subsidiary	30.5	32.3
Long-term debt, including current portion	$ 14,573.8	$ 14,343.2