Income Tax Matters - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2017	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Statutory federal income tax rate		34.00%
Tax Cuts and Jobs Act of 2017, reclassification from AOCI to retained earnings,tax effect		$ 155
Tax Cuts and Jobs Act of 2017, provisional income tax expense	$ 806
Scenario Plan [Member]
Income Tax Disclosure [Line Items]
Statutory federal income tax rate		21.00%

Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I)
-URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

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Data Type:	num:percentItemType
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Period Type:	duration