Summary of Held-to-Maturity and Available-for-Sale Securities Having Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Securities
Schedule of Investments [Line Items]
Temporarily Impaired Available-for-Sale Securities, Less than Twelve Months Fair Value	$ 1,681,483	$ 895,352
Temporarily Impaired Available-for-Sale Securities, Less than Twelve Months Unrealized Loss	25,716	4,347
Temporarily Impaired Available-for-Sale Securities, Twelve Months or Longer Fair Value	108,748	108,144
Temporarily Impaired Available-for-Sale Securities, Twelve Months or Longer Unrealized Loss	9,160	10,099
Temporarily Impaired Available-for-Sale Securities, Total Fair Value	1,790,231	1,003,496
Temporarily Impaired Available-for-Sale Securities, Total Unrealized Loss	34,876	14,446
Debt Securities \| GSE certificates
Schedule of Investments [Line Items]
Temporarily Impaired Available-for-Sale Securities, Less than Twelve Months Fair Value	876,096	232,546
Temporarily Impaired Available-for-Sale Securities, Less than Twelve Months Unrealized Loss	14,756	535
Temporarily Impaired Available-for-Sale Securities, Twelve Months or Longer Fair Value	19,848	20,440
Temporarily Impaired Available-for-Sale Securities, Twelve Months or Longer Unrealized Loss	1,108	664
Temporarily Impaired Available-for-Sale Securities, Total Fair Value	895,944	252,986
Temporarily Impaired Available-for-Sale Securities, Total Unrealized Loss	15,864	1,199
Debt Securities \| GSE debentures
Schedule of Investments [Line Items]
Temporarily Impaired Available-for-Sale Securities, Less than Twelve Months Fair Value	378,983	333,045
Temporarily Impaired Available-for-Sale Securities, Less than Twelve Months Unrealized Loss	6,732	2,665
Temporarily Impaired Available-for-Sale Securities, Total Fair Value	378,983	333,045
Temporarily Impaired Available-for-Sale Securities, Total Unrealized Loss	6,732	2,665
Debt Securities \| GSE CMOs
Schedule of Investments [Line Items]
Temporarily Impaired Available-for-Sale Securities, Less than Twelve Months Fair Value	199,397	118,694
Temporarily Impaired Available-for-Sale Securities, Less than Twelve Months Unrealized Loss	3,459	826
Temporarily Impaired Available-for-Sale Securities, Total Fair Value	199,397	118,694
Temporarily Impaired Available-for-Sale Securities, Total Unrealized Loss	3,459	826
Debt Securities \| U.S. Treasury obligations
Schedule of Investments [Line Items]
Temporarily Impaired Available-for-Sale Securities, Less than Twelve Months Fair Value	199,430	199,898
Temporarily Impaired Available-for-Sale Securities, Less than Twelve Months Unrealized Loss	248	62
Temporarily Impaired Available-for-Sale Securities, Total Fair Value	199,430	199,898
Temporarily Impaired Available-for-Sale Securities, Total Unrealized Loss	248	62
Debt Securities \| Municipal bonds
Schedule of Investments [Line Items]
Temporarily Impaired Available-for-Sale Securities, Less than Twelve Months Fair Value	10,901	11,169
Temporarily Impaired Available-for-Sale Securities, Less than Twelve Months Unrealized Loss	469	259
Temporarily Impaired Available-for-Sale Securities, Twelve Months or Longer Fair Value	40,156	41,054
Temporarily Impaired Available-for-Sale Securities, Twelve Months or Longer Unrealized Loss	1,242	542
Temporarily Impaired Available-for-Sale Securities, Total Fair Value	51,057	52,223
Temporarily Impaired Available-for-Sale Securities, Total Unrealized Loss	1,711	801
Debt Securities \| Capital trust notes
Schedule of Investments [Line Items]
Temporarily Impaired Available-for-Sale Securities, Twelve Months or Longer Fair Value	37,385	35,105
Temporarily Impaired Available-for-Sale Securities, Twelve Months or Longer Unrealized Loss	6,363	8,632
Temporarily Impaired Available-for-Sale Securities, Total Fair Value	37,385	35,105
Temporarily Impaired Available-for-Sale Securities, Total Unrealized Loss	6,363	8,632
Equity securities
Schedule of Investments [Line Items]
Temporarily Impaired Available-for-Sale Securities, Less than Twelve Months Fair Value	16,676
Temporarily Impaired Available-for-Sale Securities, Less than Twelve Months Unrealized Loss	52
Temporarily Impaired Available-for-Sale Securities, Twelve Months or Longer Fair Value	11,359	11,545
Temporarily Impaired Available-for-Sale Securities, Twelve Months or Longer Unrealized Loss	447	261
Temporarily Impaired Available-for-Sale Securities, Total Fair Value	28,035	11,545
Temporarily Impaired Available-for-Sale Securities, Total Unrealized Loss	$ 499	$ 261

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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.

+ Details

Name:	us-gaap_ScheduleOfInvestmentsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_MunicipalBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateNoteSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: