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Debt - Senior Notes - Amendments (Details) - Unsecured Senior Notes - USD ($)
Mar. 31, 2017
Apr. 19, 2018
Dec. 31, 2017
Feb. 01, 2017
Dec. 31, 2016
Apr. 01, 2016
Dec. 31, 2015
Nov. 02, 2015
Apr. 29, 2015
Debt Instrument [Line Items]                  
Debt covenant, permitted indebtedness       $ 135,000,000   $ 145,000,000   $ 125,000,000  
Debt instrument, interest rate           6.50%     5.50%
Consent fee $ 200,000   $ 200,000   $ 300,000.0 $ 300,000 $ 0    
Legal fees $ 100,000                
Subsequent Event                  
Debt Instrument [Line Items]                  
Consent fee   $ 300,000              
Interest rate if Form 10-K is filed on or before September 30, 2018   7.50%              
Interest rate if Form 10-K has not been filed on or before September 30, 2018   8.50%              
Interest rate subsequent to Form 10-K filing   7.50%