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Income Taxes - Reconciliation of Income Taxes at Federal Statutory Income Tax Rate (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation Of Income Taxes [Line Items]      
Income tax expense computed at federal statutory income tax rate $ 222.2 $ 203.7 $ 161.0
Other income taxes, net of federal tax benefit 13.4 12.6 9.4
Foreign taxes at a different rate than U.S. federal statutory income tax rate (8.3) (7.6) (8.7)
Tax benefit on income attributable to domestic production activities (10.9) (13.0) (12.5)
Net adjustments for uncertain tax positions 11.6 13.2 4.7
Share-based compensation (ASU 2016-09) (23.9) (27.8)  
Tax Act impact (25.7)    
Valuation allowance increase (decrease) (5.2) (2.1) 0.8
Miscellaneous other, net (11.7) (8.2) (1.5)
Income tax expense as reported $ 159.5 $ 169.7 $ 153.4
Effective income tax rate 25.10% 29.20% 33.40%
State and Local Jurisdiction [Member]      
Reconciliation Of Income Taxes [Line Items]      
Tax Reform deferred tax impact of tax rate changes $ (2.0) $ (1.1) $ 0.2