Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Earnings Per Share (Detail)

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Summary of Significant Accounting Policies - Earnings Per Share (Detail) - shares
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Accounting Policies [Abstract]        
Number of shares of common stock received for each exchangeable share (in shares)     1  
Weighted average shares outstanding attributable to participating securities (in shares) 2,600,000 2,300,000 2,500,000 2,500,000