Borrowings - Schedule of Debt (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 769,316,000
Advances from federal home loan banks	$ 245,117	$ 257,359
Federal Home Loan Bank, Advances, Maturing Through 2033
Debt Instrument [Line Items]
Weighted average interest rate	2.06%	1.81%
Advances from federal home loan banks	$ 245,117	$ 222,943
Federal Home Loan Bank, Advances, Maturing Through 2033 \| Minimum
Debt Instrument [Line Items]
Fixed interest rate	1.17%	1.05%
Federal Home Loan Bank, Advances, Maturing Through 2033 \| Maximum
Debt Instrument [Line Items]
Fixed interest rate	5.24%	5.24%
Federal Home Loan Bank, Advances, Open Repo Borrowings
Debt Instrument [Line Items]
Fixed interest rate		1.54%
Maximum borrowing capacity		$ 150,000
Advances from federal home loan banks	$ 0	$ 34,416

Amount of borrowings as of the balance sheet date from the Federal Home Loan Bank, which are primarily used to cover shortages in the required reserve balance and liquidity shortages.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 45
-Paragraph 1
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Amount of maximum advances or credit lines available from the Federal Home Loan Bank (FHLB).

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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

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-URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

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Interest rate of advances made and reported by Federal Home Loan Bank (FHLBank).

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