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Income Taxes - Additional Information (Detail) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Operating Loss Carryforwards [Line Items]        
U.S. federal income tax rate   21.00%    
Tax reform adjustments to provision for income taxes $ 0      
Gross tax credits       $ (888,000)
Federal net operating loss carryforward expiration year   2033    
State net operating loss carryforward expiration year   2018    
Valuation allowance related to tax law changes 2,300,000 $ 2,300,000    
Interest and penalties associated with unrecognized tax benefits     $ 100,000  
Accrued interest and penalties associated with unrecognized tax benefits 200,000 200,000 200,000  
State and Local Jurisdiction [Member]        
Operating Loss Carryforwards [Line Items]        
State NOL carryforwards 3,400,000 3,400,000 0 400,000
Alternative minimum tax (ATM) remaining federal tax NOL carryforwards 667,900,000 667,900,000    
Portion of state NOL carryforwards, not able to be utilized before expiration 608,900,000 608,900,000 536,000,000 312,300,000
Valuation allowance for deferred assets associated with carryforwards 31,000,000 31,000,000 26,900,000  
Deferred tax assets valuation allowance   4,112,000 $ 7,651,000 $ 11,855,000
Federal [Member]        
Operating Loss Carryforwards [Line Items]        
Deferred tax assets valuation allowance   14,604,000    
Internal Revenue Service (IRS) [Member]        
Operating Loss Carryforwards [Line Items]        
Gross tax credits   $ 8,100,000    
Tax carryforward period   20 years    
Tax credit carryforward expiration year   2034    
Alternative minimum tax (ATM) remaining federal tax NOL carryforwards $ 577,300,000 $ 577,300,000