Description of Business and Summary of Significant Accounting Policies - Additional Information (Detail) - Accounting Standards Update 2016-01 [Member]	9 Months Ended
Sep. 30, 2018 USD ($)
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Additional income from changes in equity investments	$ 41,200
Retained Earnings [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Effect of change in accounting principle	(618,000)
Accumulated Other Comprehensive Income [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Effect of change in accounting principle	$ 618,000

Amount of cumulative effect of the change in accounting principle or new accounting pronouncement on retained earnings or other components of equity or net assets in the statement of financial position as of the beginning of the earliest period presented.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64853466&loc=d3e22580-107794

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64853466&loc=d3e22583-107794

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=64853466&loc=d3e22499-107794

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