Federal Home Loan Bank Advances - Additional Information (Detail)	Sep. 30, 2018	Dec. 31, 2017
Federal Home Loan Bank, Advances [Line Items]
Weighted-average rates of FHLB advances	2.34%	1.55%
Federal Home Loan Bank Advances [Member]
Federal Home Loan Bank, Advances [Line Items]
Percentage of borrowings collateralized by investment securities and residential real estate loans	140.00%

The calculated weighted average interest rate for Federal Home Loan Bank advances outstanding as of the Balance Sheet date.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13(3)(a))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Name:	us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateAtPeriodEnd
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant