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Note Net income (loss) per common share - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Mar. 31, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Earnings Per Share, Basic and Diluted, Other Disclosures [Abstract]          
Accelerated Share Repurchases Final Price Paid Per Share   $ 40.6      
Stock Repurchase Program Authorized Amount   $ 75      
Total Common Shares Repurchased Accelerated Share Repurchase Agreements   1,847,372      
Employee Stock Option          
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]          
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount 0   0 0 0