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Fair Value Measurements Estimated Fair Value of Other Financial Assets And Liabilities (Tables) - USD ($)
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash, Cash Equivalents and Restricted Cash $ 320,378,000 $ 242,228,000 $ 225,414,000 $ 258,887,000
Investment Owned, at Cost 4,300,000      
Financing Receivable, Gross 2,366,000      
Debt, Long-term and Short-term, Combined Amount 539,834,000      
Fair Value, Inputs, Level 1 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents, Fair Value Disclosure 320,378,000      
Notes Receivable, Fair Value Disclosure 0      
Debt Instrument, Fair Value Disclosure 0      
Fair Value, Inputs, Level 2 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents, Fair Value Disclosure 0      
Notes Receivable, Fair Value Disclosure 2,335,000      
Debt Instrument, Fair Value Disclosure 579,776,000      
Fair Value, Inputs, Level 3 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents, Fair Value Disclosure 0      
Notes Receivable, Fair Value Disclosure 0      
Debt Instrument, Fair Value Disclosure $ 0      
Three Point Zero Percentage Convertible Notes [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage 3.00%      
Three Point Two Five Percentage Convertible Notes [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage 3.25%