SIGNIFICANT ACCOUNTING POLICIES - CAPITAL CONSTRUCTION FUND (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2017	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Accounts receivable, net		$ 198.6		$ 189.5
Cash Deposits Into CCF	$ 0.0	12.2	$ 12.5
Qualifying withdrawal from CCF	$ 0.0	43.4	$ 12.5
Eligible Accounts Receivable Assigned to CCF
Accounts receivable, net		$ 175.3		174.7
Long-term Assets | Cash
Capital Construction Fund - cash on deposit				$ 31.2