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Convertible Debentures and Credit Arrangements - Convertible Senior Debentures Narrative (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Nov. 30, 2012
Debt Instrument [Line Items]            
Deferred finance costs, noncurrent, net         $ 800,000  
Convertible Senior Debentures due 2018            
Debt Instrument [Line Items]            
Aggregate face value of convertible senior debentures $ 55,000,000   $ 55,000,000     $ 55,000,000.0
Interest rate on debentures           5.25%
Fair value of debentures outstanding 56,700,000   56,700,000      
Gross carrying amount of equity component 6,400,000   6,400,000      
Unamortized discount 2,300,000   2,300,000      
Deferred finance costs, noncurrent, net 600,000   600,000      
Carrying value of liability component 52,100,000   52,100,000      
Amortization of debt discount (premium) $ 400,000 $ 400,000 $ 1,200,000 $ 1,100,000    
Debt instrument, interest rate, effective percentage 8.70%   8.70%