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Accumulated Other Comprehensive Income - Additional Information (Detail) - USD ($)
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Other comprehensive income taxes $ 0  
Actuarial gain related to pension, tax effect 400,000 $ 600,000
Accounting Standards Update 2016-01 [Member]    
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Amounts reclassified from accumulated other comprehensive income $ 31,000,000