Quarterly report pursuant to Section 13 or 15(d)

Subsequent Events - Additional Information (Detail)

v3.8.0.1
Subsequent Events - Additional Information (Detail) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 1 Months Ended 3 Months Ended 9 Months Ended
Oct. 31, 2017
Dec. 31, 2017
Sep. 30, 2017
Sep. 30, 2016
Subsequent Event [Line Items]        
Reduction in cash and cash equivalents     $ (792.3) $ 101.5
Dutch Auction Tender Offer [Member] | Scenario, Forecast [Member]        
Subsequent Event [Line Items]        
Reduction in cash and cash equivalents   $ 457.8    
Reduction in shareholders' equity   $ 457.8    
Subsequent Event [Member] | Dutch Auction Tender Offer [Member]        
Subsequent Event [Line Items]        
Repurchase of common stock, shares 6.7      
Repurchase of common stock, value $ 457.8      
Average cost per share of shares repurchased $ 68.00      
Non-transferable contractual contingent value right agreement initial fair value $ 7.3      
Subsequent Event [Member] | Dutch Auction Tender Offer [Member] | Maximum [Member]        
Subsequent Event [Line Items]        
Non-transferable contractual contingent value right agreement, going private transaction term 2 years