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Fair Value Option - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Feb. 22, 2018
Dec. 31, 2017
Sep. 30, 2017
Fair Value Option Qualitative Disclosures Related To Election [Line Items]          
Debt instrument, interest rate, stated percentage 5.75% 5.75%      
Long term debt, fair value $ 55,878 $ 55,878   $ 0  
Principal outstanding balance [1] $ 691,638 $ 691,638   $ 800,842  
5.75% Senior Subordinated Notes Due August 2023          
Fair Value Option Qualitative Disclosures Related To Election [Line Items]          
Debt instrument, interest rate, stated percentage 5.75% 5.75%      
Convertible Notes          
Fair Value Option Qualitative Disclosures Related To Election [Line Items]          
Debt instrument, interest rate, stated percentage 1.50% 1.50%      
Convertible Notes | 5.75% Senior Subordinated Notes Due August 2023          
Fair Value Option Qualitative Disclosures Related To Election [Line Items]          
Debt instrument, interest rate, stated percentage 5.75% 5.75% 5.75%   5.75%
Long term debt, fair value $ 55,900 $ 55,900      
Principal outstanding balance 111,000 111,000      
Change in fair value of convertible notes $ 17,200 $ 73,700      
[1] Carrying amounts include aggregate unamortized debt discount and debt issuance costs.