Analysis of Contractual Maturities of Outstanding Repurchase Agreements (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amount
Repurchase agreements	$ 450,000	$ 1,500,000
Weighted Average Interest Rate
Contractual Maturity Over 90 days, Weighted Average Interest Rate	2.44%
Contractual Maturity
Amount
2018	$ 250,000
2019	200,000
Repurchase agreements	$ 450,000
Weighted Average Interest Rate
2018	3.04%
2019	1.69%
Contractual Maturity Over 90 days, Weighted Average Interest Rate	2.44%

Securities Sold under Agreements to Repurchase, Maturities Summary [Abstract]

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Amount of cash received as security in return for loaning securities to another party which are due within one year of the Balance Sheet date.

+ References

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Securities Sold Under Agreements To Repurchase Maturities Summary Due Within Two Years Of Balance Sheet Date

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Securities Sold under Agreements to Repurchase, Maturities Summary, Weighted Average Interest Rate of Amounts Due [Abstract]

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The weighted average interest rate applicable to the cash received as security in return for loaning securities to another party which are due within one year of the Balance Sheet date.

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- Definition

The weighted average interest rate applicable to the cash received as security in return for loaning securities to another party which are due from one to two years of the Balance Sheet date.

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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://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)(i)(A)(ii))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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

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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))
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section 55
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-Topic 210
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-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=49176635&loc=d3e9760-107771

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