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Income Taxes - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2016
Income Tax [Line Items]          
Uncertain tax positions $ 0   $ 0 $ 0 $ 0
Federal corporate tax rate 21.00%   31.55%    
Provisional benefit for re-measurement of deferred tax liabilities   $ 61,000,000      
Additional benefit due to re-measurement of deferred tax liabilities $ 700,000        
Total benefit due to re-measurement of deferred tax liabilities     $ 61,700,000    
State Income Tax [Member]          
Income Tax [Line Items]          
Net operating loss carryforward 1,500,000   1,500,000    
Investment credits $ 14,900,000   $ 14,900,000