SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Cash, cash equivalents and restricted cash (Details) - USD ($) $ in Millions |
Dec. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|---|---|---|---|---|
| Cash, cash equivalents and restricted cash | ||||
| Cash and cash equivalents | $ 223.5 | $ 215.1 | $ 388.0 | |
| Restricted cash included in other current assets | 10.2 | 10.2 | 10.2 | |
| Total cash, cash equivalents, and restricted cash | $ 233.7 | $ 225.3 | $ 398.2 | $ 249.6 |