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Income Taxes - Additional Information (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Jun. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2017
Income Tax Disclosure [Line Items]                
U.S. federal corporate tax rate       35.00%        
Provisional tax expense $ 46.5     $ 46.5        
Discrete tax expense       54.4        
Non-discrete tax benefit       $ (7.9)        
Impact of annual effective tax rate       12.60%        
Discrete tax provision 5.1     $ 5.1        
Increase in income tax expense, net of foreign tax credits       49.3        
Provision for income taxes 53.3 $ 6.6   56.5 $ 7.6      
Gross unrecognized tax benefits 18.0     18.0       $ 15.2
Gross unrecognized tax benefits increased during the year       2.8        
Interest and penalties accrued related to unrecognized tax benefits 1.3     1.3        
Increase in interest and penalties accrued related to unrecognized tax benefits       0.1        
Cumulative-effect adjustment related to retained earnings, tax       8.3        
Maximum [Member]                
Income Tax Disclosure [Line Items]                
Estimated increase in unrecognized tax benefit in next twelve months $ 2.4     2.4        
Undistributed Foreign Earnings [Member]                
Income Tax Disclosure [Line Items]                
Provision for income taxes       $ 0.0        
Cash Assets [Member]                
Income Tax Disclosure [Line Items]                
One-time transition tax rate on post-1986 foreign earnings and profits 15.50%     15.50%        
Other Specified Assets [Member]                
Income Tax Disclosure [Line Items]                
One-time transition tax rate on post-1986 foreign earnings and profits 8.00%     8.00%        
Scenario, Forecast [Member]                
Income Tax Disclosure [Line Items]                
U.S. federal corporate tax rate     21.00%     21.00% 28.17%