Income Taxes - Additional Information Regarding Examinations (Detail)	3 Months Ended			12 Months Ended
	Dec. 31, 2017	Sep. 30, 2015	Mar. 31, 2011	Jun. 30, 2018
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