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Convertible Senior Notes Components of consolidated balance sheets associated with convertible senior notes (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Nov. 30, 2005
May 31, 2005
Debt Instrument [Line Items]        
Convertible Debt $ 61,393,000 $ 60,791,000    
Carrying amount of equity component included in additional paid-in capital 108,714,000 108,714,000    
Excess of instruments’ if-converted values over face principal amounts 0 0    
5.875% Convertible Senior Notes Due 2035 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount 88,280,000 88,280,000 $ 300,000,000.0  
Debt Instrument, Unamortized Discount $ 26,887,000 $ 27,489,000    
Interest rate on notes     5.875% 5.875%