Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Fair value equity transfers in and out of level 3 instruments	$ 0	$ 0	$ 0
Fair value equity transfers between level 1 and level 2 instruments	$ 0	$ 0	$ 0

Amount of transfers of financial instruments classified in shareholders' equity measured on a recurring basis out of level 1 of the fair value hierarchy into level 2.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

Name:	us-gaap_FairValueEquityLevel1ToLevel2TransfersAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant