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Income Per Share Information - Additional Information (Detail) - shares
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 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 384,775 1,972,360 1,141,522 1,704,848