CAPITAL CONSTRUCTION FUND (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Maximum period to commit fund deposits for qualified purposes	25 years
Period over which deposits will be treated as non-qualified withdrawals	5 years
Capital Construction Fund - cash on deposit	$ 0.9	$ 31.2
Accounts receivable, net	194.6	189.5
Cash Deposits Into CCF	171.4	123.4	$ 77.9
Qualifying withdrawal from CCF	201.7	92.2	$ 105.4
Eligible Accounts Receivable Assigned to CCF
Accounts receivable, net	134.8	174.7
Long-term Assets | Cash
Capital Construction Fund - cash on deposit	$ 0.9	$ 31.2