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Convertible Senior Notes Narrative (Details) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Nov. 30, 2005
May 31, 2005
Debt Instrument [Line Items]          
Own Share Lending Arrangement, Duration of Share Lending Arrangement   30 years      
Own Share Lending Arrangement, Value Received       $ 0.001  
Own-share Lending Arrangement, Shares, Outstanding, Value   $ 3,500,000.000      
Debt Instrument, Convertible, Conversion Ratio   4063.00%      
Debt Conversion, Converted Instrument, Amount   $ 1,000      
Own-share Lending Arrangement, Shares, Outstanding 1,459,233 1,459,233 1,459,233 5,677,950  
Maturity date   Nov. 30, 2035      
Conversion price for convertible senior notes (in dollars per share)   $ 24.61      
Gains (Losses) on Extinguishment of Debt   $ 0 $ 1,151,000    
5.875% Convertible Senior Notes Due 2035 [Member]          
Debt Instrument [Line Items]          
Aggregate principal amount $ 88,280,000 $ 88,280,000 88,280,000 $ 300,000,000.0  
Interest rate on notes       5.875% 5.875%
Tender Offer Repurchase Amount - Face Value     $ 5,000,000    
Conversion price for convertible senior notes (in dollars per share)   $ 24.61      
Tender Offer Repurchase Amount 2.3        
Gains (Losses) on Extinguishment of Debt $ 1,200,000