Income Taxes - Additional Information (Detail) - USD ($)	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Deferred income tax assets	$ 46,970,000	$ 47,679,000
Valuation allowance	0	$ 0
Decrease in deferred tax assets	$ (700,000)

Amount, after allocation of valuation allowances and deferred tax liability, of deferred tax asset attributable to deductible differences and carryforwards, with jurisdictional netting.

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

Name:	us-gaap_DeferredIncomeTaxAssetsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant