Maturities of Collateral Underlying Reverse Repurchase Agreements and Securities Lending Transactions (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	$ 35,454	$ 36,337
Securities lending transactions	141,571	203,909
Total Borrowings	177,025	240,246
Gross amount of recognized liabilities for reverse repurchase agreements and securities lending included on the Balance Sheets	176,767	246,417
Residential mortgage-backed securities
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	35,454	36,337
Corporate securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	44,264	34,493
Equity AFS securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	3,508	405
Maturity Overnight and Continuous
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	141,571	203,909
Total Borrowings	141,571	203,909
Maturity Overnight and Continuous \| Corporate securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	44,264	34,493
Maturity Overnight and Continuous \| Equity AFS securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	3,508	405
Maturity up to 30 Days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	35,454	36,337
Total Borrowings	35,454	36,337
Maturity up to 30 Days \| Residential mortgage-backed securities
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	35,454	36,337
US Government Corporations and Agencies Securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	93,799	169,011
US Government Corporations and Agencies Securities \| Maturity Overnight and Continuous
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	$ 93,799	$ 169,011

Gross Recognized Liabilities for Repurchase Agreements and Securities Lending

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Carrying value of securities sold under agreements to repurchase and deposits received for securities loaned.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13(1))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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- Definition

The amount of cash received as security in return for loaning securities to another party.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(11))
-URI http://asc.fasb.org/extlink&oid=6488278&loc=d3e603758-122996

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- Definition

The carrying value of securities purchased to be resold in reverse repurchase transactions as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(2)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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