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DEBT - 2017 Convertible, Term Loan Facility, and Margin Loan - Narrative (Details)
1 Months Ended 3 Months Ended 12 Months Ended
Mar. 03, 2017
USD ($)
shares
Feb. 17, 2017
USD ($)
shares
$ / shares
Feb. 13, 2017
$ / shares
Dec. 31, 2020
USD ($)
Aug. 31, 2019
USD ($)
Oct. 31, 2018
Mar. 31, 2019
$ / shares
Dec. 31, 2020
USD ($)
extensionOption
Mar. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
shares
Jul. 31, 2018
USD ($)
Mar. 31, 2012
Debt Instrument [Line Items]                          
Cash collateral       $ 163,181,000       $ 163,181,000   $ 188,289,000      
Margin loan facility | Line of Credit                          
Debt Instrument [Line Items]                          
Cash collateral       0       0   10,000,000      
Convertible Debt | 2017 Convertible Bonds                          
Debt Instrument [Line Items]                          
Face amount   $ 402,500,000                      
Coupon rate   2.75%                      
Debt instrument conversion ratio   0.0265308         26.993            
Debt instrument conversion price (in dollars per share) | $ / shares   $ 37.69         $ 37.05            
Debt instrument threshold percentage of stock price trigger   35.00%                      
Share price (in dollars per share) | $ / shares     $ 27.92                    
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%                      
Proceeds from issuance of convertible bonds   $ 360,200,000                      
Convertible debt   320,300,000                      
Convertible, carrying amount of equity component   $ 39,900,000                      
Distribution (in dollars per share) | $ / shares             $ 0.150            
Credit Facility | Term Facility                          
Debt Instrument [Line Items]                          
Face amount       $ 100,000,000.0 $ 150,000,000     $ 100,000,000.0   $ 0      
Maturity period of debt       366 days 15 months                
Basis spread on variable rate       5.00%                  
Repayments of lines of credit       $ 50,000,000.0                  
Credit Facility | Term Facility | Extended Maturity                          
Debt Instrument [Line Items]                          
Maturity period of debt               366 days          
Number of loan extension options | extensionOption               2          
Credit Facility | Margin loan facility                          
Debt Instrument [Line Items]                          
Face amount         $ 110,000,000.0                
Secured Debt                          
Debt Instrument [Line Items]                          
Maturity period of debt           5 years   5 years          
Secured Debt | Margin loan facility                          
Debt Instrument [Line Items]                          
Coupon rate                         3.75%
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                   21,226,586    
Facility remaining until maturity                 $ 30,000,000.0     $ 100,000,000.0  
LIBOR | Credit Facility | Margin loan facility                          
Debt Instrument [Line Items]                          
Basis spread on variable rate         2.75%                
LIBOR | Secured Debt | Margin loan facility                          
Debt Instrument [Line Items]                          
Basis spread on variable rate 3.95%             2.95%          
Minimum | LIBOR | Credit Facility | Term Facility                          
Debt Instrument [Line Items]                          
Basis spread on variable rate         1.50%                
Maximum | LIBOR | Credit Facility | Term Facility                          
Debt Instrument [Line Items]                          
Basis spread on variable rate         2.75%