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Income Per Share Information - Additional Information (Detail) - shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Contingently Issuable Shares [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Shares excluded in computation of diluted income per share 259,274 621,805 1,833,941