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Income Per Share Information - Additional Information (Detail) - shares
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 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 1,317,651 1,536,189 2,037,886 1,553,158