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DEBT - DESCRIPTION (Details) - USD ($) $ in Millions				6 Months Ended
DEBT - DESCRIPTION (Details) - USD ($) $ in Millions	Jun. 22, 2020	May 12, 2020	Apr. 27, 2020	Jun. 30, 2020	Dec. 31, 2019
Debt
Payment of debt financing costs				$ 18.5
Proceeds from issuance of debt				$ 325.5
Revolving Credit Facility
Debt
Interest rate during period (as a percent)				3.25%
Unused portion of credit facility				$ 433.2
Standby and commercial letters of credit
Debt
Standby letters of credit				$ 8.1
Private Placement Term Loans
Debt
Repayment of debt		$ 169.5
5.34%, payable through 2028
Debt
Interest rate (as a percent)				5.34%	5.34%
5.79%, payable through 2020
Debt
Interest rate (as a percent)				5.79%	5.79%
5.27%, payable through 2029
Debt
Interest rate (as a percent)				5.27%	5.27%
4.35%, payable through 2044
Debt
Interest rate (as a percent)				4.35%	4.35%
Title XI Notes \| MatNav
Debt
Debt issued	$ 139.6		$ 185.9
Interest rate (as a percent)	1.35%		1.22%
Annual principal payments	$ 3.0		$ 4.0
Payment of debt financing costs	6.7		8.7
Proceeds from issuance of debt	$ 132.9		$ 177.0