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Cash and Investments in Marketable Securities - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Cash and Investments in Marketable Securities [Line Items]          
Maximum maturity term for debt securities investment     Two years or less    
Weighted average maturity term for debt securities investment     One year or less    
Debt investment maturity period greater than one year $ 59,596,000   $ 59,596,000   $ 0
Available-for-sale securities, sold 0 $ 5,000,000 8,823,000 $ 4,969,000  
Level 1 to level 2 transfers 0 0 0 0  
Level 2 to level 1 transfers 0 $ 0 0 $ 0  
Senior secured notes, principal amount $ 250,000,000   $ 250,000,000    
Senior Notes [Member] | 7.75% Senior Secured Notes Due October 2020 [Member]          
Cash and Investments in Marketable Securities [Line Items]          
Senior secured notes, interest rate 7.75%   7.75%    
Senior secured notes, maturity date     Oct. 05, 2020    
Minimum cash and investments in marketable securities to be maintained $ 60,000,000   $ 60,000,000    
Senior Notes [Member] | 7.75% Senior Secured Notes Due October 2020 [Member] | Level 3 [Member]          
Cash and Investments in Marketable Securities [Line Items]          
Senior secured notes, interest rate 7.75%   7.75%    
Senior secured notes, maturity date     Oct. 05, 2020    
Senior secured notes, principal amount $ 250,000,000   $ 250,000,000