Stock - Based Compensation - Summary of Accumulated Stock Units (Detail) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Accumulated stock units	138,768	140,044
Average price	$ 22.84	$ 20.87

Deferred compensation arrangement with individual accumulated stock units For deferred board fees.

No definition available.

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant