Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements Fair Value Measurements (Assets and Liabilities Measured on a Non-recurring Basis) (Details)

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Fair Value Measurements Fair Value Measurements (Assets and Liabilities Measured on a Non-recurring Basis) (Details) - USD ($)
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Cash, Cash Equivalents and Restricted Cash $ 275,504,000 $ 242,228,000 $ 209,799,000 $ 258,887,000
Investment Owned, at Cost 4,300,000      
Financing Receivable, Gross 2,507,000      
Debt, Long-term and Short-term, Combined Amount 573,497,000      
Fair Value, Inputs, Level 1 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Cash and Cash Equivalents, Fair Value Disclosure 275,504,000      
Notes Receivable, Fair Value Disclosure 922,000      
Debt Instrument, Fair Value Disclosure 0      
Fair Value, Inputs, Level 2 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Cash and Cash Equivalents, Fair Value Disclosure 0      
Notes Receivable, Fair Value Disclosure 1,542,000      
Debt Instrument, Fair Value Disclosure 590,968,000      
Fair Value, Inputs, Level 3 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [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, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage 3.00%