Loans, Net - Additional Information (Detail) - USD ($)	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Pledged loans as collateral for FHLB advances, carrying amount	$ 3,380,000,000		$ 3,360,000,000
Loans	8,704,392,000		7,977,784,000
Purchase of loans from third parties	1,100,000.0	$ 0
Sale of loans to third parties	18,300,000	120,200,000
Reclassification between non-accretable and accretable discount	100,000	$ (5,300,000)
ASC 310-30 Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principle balance of ASC 310-30 loans	293,600,000		183,900,000
1-4 single family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments of consumer mortgage loans secured by residential real estate properties for which formal foreclosure proceedings are in process	2,600,000
Syndicated National Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 295,700,000		$ 289,100,000

Loans And Leases Receivable Unpaid Principal Balance

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

The unpaid principal balance of loans purchased from a third party during the period.

+ References

No definition available.

+ Details

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- Definition

The unpaid principal balance of loans sold to a third party during the period.

+ References

No definition available.

+ Details

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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.

+ Details

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- Definition

Amount of increase (decrease) of the excess of a loan's cash flows expected to be collected over the investor's initial investment in acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82894507&loc=d3e13503-111538

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

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- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

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Period Type:	instant

- Definition

Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL49126937-111524

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- Definition

The carrying amount, as of the date of the latest financial statement presented, of loans receivable which are owned but transferred to serve as collateral for the payment of FHLB debt obligations, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

+ Details

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Data Type:	xbrli:monetaryItemType
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- Details

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- Details

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- Details

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