Quarterly report pursuant to Section 13 or 15(d)

Shareholders' Equity - Additional Information (Detail)

v3.8.0.1
Shareholders' Equity - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Feb. 28, 2014
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Sep. 30, 2016
Mar. 31, 2014
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Feb. 21, 2017
Stockholders Equity [Line Items]                    
Share repurchase program authorized amount                   $ 1,500.0
Share repurchase program, remaining authorized capacity                 $ 232.9  
Share repurchase program expiration date             Feb. 21, 2020   Jun. 30, 2017  
Retained earnings (accumulated deficit)   $ 240.7         $ 240.7   $ (66.3)  
Additional paid-in-capital   $ (452.0)         (452.0)   $ (467.6)  
Repurchase of common stock, value             $ 346.2 $ 12.5    
Treasury stock shares, at cost   4,600,000         4,600,000      
Shares repurchases, value             $ 299.2 0.0    
Withheld for tax purpose for share-based compensation plans             47.0 12.5    
Other comprehensive income (loss) before foreign currency translation adjustments reclassifications, Tax expense (benefits)   $ (1.8)     $ 5.8   3.5 7.3    
Other comprehensive Unrealized Gain (Loss) on Derivatives before reclassifications, Tax expense (benefits)               (0.3)    
Amounts reclassified from accumulated other comprehensive income (loss) to income, tax expense (benefits), other               0.1    
Capped call transactions [Member]                    
Stockholders Equity [Line Items]                    
Capped call transactions with financial institutions $ 123.8                  
Capped call transactions price per common share $ 120.79                  
Increase (decrease) in additional paid-in capital, other           $ (123.8)        
Capped call transactions [Member] | Convertible Notes [Member]                    
Stockholders Equity [Line Items]                    
Convertible notes initial conversion price $ 86.28                  
Prepaid forward share repurchase transactions [Member]                    
Stockholders Equity [Line Items]                    
Share repurchase program, remaining authorized capacity   1,200.8         $ 1,200.8      
Forward share repurchase transactions amount $ 685.8         685.8        
Forward transactions, shares to be purchased 9,900,000                  
Average cost per share of shares repurchased $ 69.02                  
Settlement date of forward transactions             Aug. 15, 2019      
Retained earnings (accumulated deficit)           (653.9)        
Additional paid-in-capital           (31.9)        
Non-cash forward transaction issuance costs           $ 35.8        
Non-cash interest expense relating to amortization of non-cash issuance costs   $ 1.6     $ 1.6   $ 4.8 $ 4.8    
Open market purchases [Member]                    
Stockholders Equity [Line Items]                    
Average cost per share of shares repurchased   $ 72.38 $ 67.06 $ 56.10            
Repurchase of common stock, shares   800,000 2,700,000 1,100,000 0     0    
Repurchase of common stock, value   $ 58.7 $ 179.8 $ 60.7            
Treasury stock shares, at cost   4,600,000         4,600,000