Income Taxes - Additional Information (Detail) $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($)
Income Tax Examination [Line Items]
Reserve on US-based deferred tax amounts	$ 52.9
Federal [Member] \| IRS [Member]
Income Tax Examination [Line Items]
Income tax years under examination	2013 2014 2015 2016 2017

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319

Name:	us-gaap_DeferredTaxAssetsValuationAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant