LONG-TERM OBLIGATIONS - Schedule of Letters of Credit Outstanding (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Letters of credit outstanding, amount	$ 115.2	$ 82.5
$400 Million Facility
Debt Instrument [Line Items]
Letters of credit outstanding, amount	0.0	0.0
$300 Million Facility
Debt Instrument [Line Items]
Letters of credit outstanding, amount	59.7	35.3
Bilateral Arrangements
Debt Instrument [Line Items]
Letters of credit outstanding, amount	$ 55.5	$ 47.2