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Debt - Senior Unsecured Notes Exchangeable (Details)
1 Months Ended 12 Months Ended
Aug. 31, 2014
USD ($)
d
$ / shares
Dec. 31, 2018
USD ($)
$ / shares
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Debt Instrument [Line Items]        
Senior unsecured notes, net   $ 1,046,219,000 $ 707,895,000  
Interest expense   79,623,000 68,473,000 $ 70,595,000
Amortization expense of deferred financing costs   $ 4,100,000 4,700,000 5,000,000
Senior unsecured notes - exchangeable | Senior unsecured notes - exchangeable        
Debt Instrument [Line Items]        
Face amount $ 250,000,000      
Stated interest rate 2.625% 2.625%    
Conversion price (USD per share) | $ / shares $ 19.45 $ 19.23    
Debt, conversion ratio 0.0514059 0.0520116    
Percentage of principal redeemed 100.00%      
Effective rate (in percentage) 3.80% 3.93%    
Senior unsecured notes, net $ 236,600,000      
Embedded conversion option   $ 13,400,000 13,400,000  
Remaining discount amortization period (in years) 5 years      
Unamortized discount   1,600,000 4,300,000  
Underwriting expense $ 3,100,000      
Deferred finance costs $ 2,900,000      
Embedded conversion feature, reduction   200,000 200,000  
Interest on note   9,900,000 9,900,000 9,900,000
Interest expense   6,600,000 6,600,000 6,600,000
Accretion of debt discount   2,700,000 2,700,000 2,700,000
Amortization expense of deferred financing costs   $ 600,000 $ 600,000 $ 600,000
Covenant scenario one | Senior unsecured notes - exchangeable | Senior unsecured notes - exchangeable        
Debt Instrument [Line Items]        
Threshold percentage of stock price trigger (in percentage) 130.00%      
Threshold trading days | d 20      
Threshold consecutive trading days | d 30      
Covenant scenario two | Senior unsecured notes - exchangeable | Senior unsecured notes - exchangeable        
Debt Instrument [Line Items]        
Threshold percentage of stock price trigger (in percentage) 98.00%      
Threshold consecutive trading days | d 5      
Threshold consecutive business days 5 days