Details of Repurchase Agreements (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Outstanding [Line Items]
Contractual Maturity Over 90 days, amount	$ 500,000	$ 450,000
Contractual Maturity Over 90 days, Weighted Average Interest Rate	2.10%
Maturity 30 to 90 Days [Member]
Debt Outstanding [Line Items]
Contractual Maturity Over 90 days, amount	$ 200,000
Contractual Maturity Over 90 days, Weighted Average Interest Rate	1.69%
Maturity Greater than 90 Days [Member]
Debt Outstanding [Line Items]
Contractual Maturity Over 90 days, amount	$ 300,000
Contractual Maturity Over 90 days, Weighted Average Interest Rate	2.37%
Mortgage-Related Securities
Debt Outstanding [Line Items]
Contractual Maturity Over 90 days, Fair Value	$ 213,135
Contractual Maturity Over 90 days, Amortized Cost	215,244
Mortgage-Related Securities \| Maturity 30 to 90 Days [Member]
Debt Outstanding [Line Items]
Contractual Maturity Over 90 days, Fair Value	213,135
Contractual Maturity Over 90 days, Amortized Cost	215,244
U.S. Treasury and GSE Obligations
Debt Outstanding [Line Items]
Contractual Maturity Over 90 days, Fair Value	317,683
Contractual Maturity Over 90 days, Amortized Cost	321,163
U.S. Treasury and GSE Obligations \| Maturity Greater than 90 Days [Member]
Debt Outstanding [Line Items]
Contractual Maturity Over 90 days, Fair Value	317,683
Contractual Maturity Over 90 days, Amortized Cost	$ 321,163

Debt Outstanding [Line Items]

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- Definition

Securities Held as Collateral, at Amortized Cost

+ References

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- Definition

Interest rate on the amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(ii))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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- Definition

The carrying amount (fair value) as of the balance sheet date of securities held that had been pledged by counterparties under financing and lending arrangements, pursuant to which the entity has the right by agreement or custom to sell or re-pledge such securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5086-111524

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- Definition

Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
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-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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-Topic 210
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-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

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-Topic 235
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