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Schedule of Amortization of Premium and Accretion of Debt Issuance Costs (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Debt Instrument [Line Items]        
Amortization of debt premium $ 375 $ 431 $ 1,072 $ 1,564
5.75% Senior convertible notes due August 15, 2018 and 5.75% Senior convertible notes due October 23, 2021        
Debt Instrument [Line Items]        
Amortization of debt premium 35 62 119 182
Accretion expense - debt issuance cost $ 2 $ 70 $ 2 $ 204