SIGNIFICANT ACCOUNTING POLICIES - CAPITAL CONSTRUCTION FUND (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Accounts receivable, net	$ 328.6	$ 328.6		$ 253.4
Cash Deposits Into CCF		31.2	$ 97.1
Withdrawals from CCF		31.2	$ 97.1
Eligible Accounts Receivable Assigned to CCF
Accounts receivable, net	1.8	1.8		$ 1.7
Cash Deposits Into CCF	0.0	31.2
Withdrawals from CCF	$ 0.0	$ 31.2