Income Tax - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Dec. 31, 2017
Effective Tax Rate Reconciling Items [Line Items]
Statutory federal income tax rate		21.00%	35.00%
Tax Cuts and Jobs Act 2017, Incomplete accounting provisional income tax expense (benefit)	$ 20.4
Accounting Standards Update 2014-09
Effective Tax Rate Reconciling Items [Line Items]
Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Deferred Tax Liability, Income Tax Benefit	$ 12.6