Performance Awards - Additional Information (Detail) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Unrecognized expense	$ 4.9
2014 EIP \| Performance Awards
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Performance period	3 years
Unrecognized expense	$ 2.0

Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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