Quarterly report pursuant to Section 13 or 15(d)

Income Taxes - Additional Information (Detail)

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Income Taxes - Additional Information (Detail) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Contingency [Line Items]      
Effective corporate income tax rate 21.00% 35.00%  
Deferred tax assets, valuation allowance $ 6.2 $ 6.2 $ 4.5
Deferred tax assets, operating loss carryforwards, state and local 6.0 6.0 4.2
Deferred tax assets, impairment losses $ 0.2 0.2 $ 0.3
Domestic Tax Authority [Member]      
Income Tax Contingency [Line Items]      
Net operating loss carryforwards for federal income tax purpose   $ 153.1  
Ending year of expiry for net operating loss carryforwards   2020  
State and Local Jurisdiction [Member]      
Income Tax Contingency [Line Items]      
Net operating loss carryforwards for federal income tax purpose   $ 790.4  
Ending year of expiry for net operating loss carryforwards   2037  
Beginning year of expiry for net operating loss carry forwards   2018