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LONG-TERM DEBT - Interest on Long-term Debt (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
LONG-TERM DEBT    
Interest expense $ 70.0 $ 63.1
Borrowing costs capitalized $ 6.4 $ 3.1
Capitalization rate (as a percent) 1.37% 1.70%
Credit Facility    
LONG-TERM DEBT    
Cash interest paid $ 0.1 $ 3.6
Cash standby fees paid 5.6 5.2
Notes    
LONG-TERM DEBT    
Cash interest paid $ 71.3 $ 59.8