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Income Taxes - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Income Tax [Line Items]          
Net deferred income tax assets         $ 50,500,000
Decrease (increase) in effective tax rate 2.70% 0.60% 5.20% (4.60%)  
U.S. corporate tax rate     21.00%   35.00%
Effective tax rate additional tax expense (benefit) associated with stock compensation       $ 1,100,000  
Decrease in unrecognized tax positions $ 1,800,000   $ 1,800,000    
Interest and penalties $ 1,800,000   $ 1,800,000   $ 1,900,000
Maximum [Member]          
Income Tax [Line Items]          
Effective tax rate additional tax expense (benefit) associated with stock compensation   $ (100,000)