Stock Options and Other Equity-Based Incentive Instruments - Summary of Compensation Costs for RSUs (Detail) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Service Condition Vesting RSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation costs for RSUs recognized in income	$ 895,000	$ 616,000	$ 458,000
Market Plus Service Condition Vesting RSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation costs for RSUs recognized in income	$ 432,000	$ 271,000	$ 179,000

The noncash expense that represents the cost of restricted stock or unit distributed to employees as compensation.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Name:	us-gaap_RestrictedStockExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration