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Goodwill - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Goodwill And Intangible Assets Disclosure [Abstract]    
Goodwill $ 1,641,197,000 $ 1,654,292,000
Impairment charge 0  
Fair value exceeded book value, amount 1,400,000,000  
Fair value exceeded book value impact after effect of new tax law, amount $ 2,400,000,000  
Fair value exceeded book value, percentage 24.00%  
Fair value exceeded book value impact after effect of new tax law, percentage 42.00%