Income Taxes - Additional Information Regarding Examinations (Detail)	3 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2015	Mar. 31, 2011
Earliest Tax Year | Foreign Country
Income Tax Examination [Line Items]
Tax years subject to examination	1996
Latest Tax Year | Foreign Country
Income Tax Examination [Line Items]
Tax years subject to examination	2017
Internal Revenue Service (IRS) | Earliest Tax Year
Income Tax Examination [Line Items]
Tax years under audit		2010	2007	2004
Tax years subject to examination	2010
Internal Revenue Service (IRS) | Latest Tax Year
Income Tax Examination [Line Items]
Tax years under audit		2013	2009	2006
Tax years subject to examination	2017