Stockholders' Equity - Additional Information (Detail) - shares	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Shares reserved for issuance under the Directors Deferred Fee Plan	228,423	253,413

Number of common shares reserved for future issuance related to deferred compensation arrangements with individuals.

No definition available.

Name:	us-gaap_DeferredCompensationArrangementWithIndividualCommonStockReservedForFutureIssuance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant