Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Taxes [Line Items]
Effective income tax rate	31.10%	29.50%	23.00%	29.20%
Increase/(decrease) in effective tax rate due to impact of discrete tax items	0.20%	2.80%	(5.30%)	3.40%
Income tax expense at federal statutory rate			21.00%		35.00%
Net decrease to income tax reserves for unrecognized tax benefits based on tax positions related to current and prior years	$ 0.2
Decrease of income tax reserves for unrecognized tax benefits	2.2
Unrecognized tax benefits reserve that would impact effective tax rate if released into income	29.8		$ 29.8
Current Tax Payable Part of Other Current Liabilities
Income Taxes [Line Items]
Unrecognized tax benefits reserve that would impact effective tax rate if released into income	4.1		4.1
Non-Current Tax Payable Part of Other Non-current Liabilities
Income Taxes [Line Items]
Unrecognized tax benefits reserve that would impact effective tax rate if released into income	$ 25.7		$ 25.7