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DEBT - Convertible Bonds Narrative (Details)
1 Months Ended 12 Months Ended
Feb. 17, 2017
USD ($)
shares
$ / shares
Feb. 13, 2017
$ / shares
Mar. 31, 2012
USD ($)
Dec. 31, 2017
USD ($)
shares
Mar. 03, 2017
USD ($)
shares
Dec. 31, 2016
shares
Convertible Debt | Convertible Bonds            
Debt Instrument [Line Items]            
Coupon rate     3.75%      
Proceeds from issuance of convertible bonds     $ 250,000,000      
Convertible Debt | 2017 Convertible Bonds            
Debt Instrument [Line Items]            
Face amount $ 402,500,000.0          
Maturity period of debt 5 years          
Coupon rate 2.75%          
Debt instrument conversion ratio 0.0265308          
Debt instrument conversion price (in dollars per share) | $ / shares $ 37.69          
Debt instrument threshold percentage of stock price trigger 35.00%          
Share Price | $ / shares   $ 27.92        
Proceeds from issuance of convertible bonds $ 360,200,000          
Convertible Debt 320,300,000          
Convertible, carrying amount of equity component 39,900,000          
Derivative, cost of hedge $ 31,200,000          
Number of shares issued if converted | shares 10,678,647          
Debt instrument capped call strike price (in usd per share) | $ / shares $ 37.69          
Cap price premium | $ / shares $ 48.86          
Capped price threshold   75.00%        
Debt instrument all-in cost percent 4.30%          
Secured Debt | Margin Loan Facility            
Debt Instrument [Line Items]            
Maturity period of debt       3 years    
Proceeds of new margin loan facility (up to)         $ 150,000,000  
Common units, pledged as collateral | shares       20,852,291 20,852,291 20,852,291
Payments of dividends and interest       $ 30,900,000    
LIBOR | Secured Debt | Margin Loan Facility            
Debt Instrument [Line Items]            
Basis spread on variable rate       3.95%