Long-Term Borrowings - Additional Information (Detail)	6 Months Ended	12 Months Ended
Jun. 30, 2018 USD ($) Trust	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Unused borrowing amount	$ 3,579,159,000
FHLB advances	$ 1,560,365,000	$ 1,271,531,000
FHLB advances, weighted-average interest rate	1.97%
FHLB advances, maximum maturity period	7 years
Number of statutory business trusts \| Trust	14
Outstanding balances of debentures	$ 234,276,000	$ 242,446,000
Maximum time to defer payment of interest on subordinate debt		5 years
Maximum [Member]
Debt Instrument [Line Items]
Risk based assets	$ 15,000,000,000
Capital Securities [Member]
Debt Instrument [Line Items]
Number of statutory business trusts \| Trust	14

The maximum remaining maturity period for advances from Federal Home Loan Banks as of the balance sheet date.

No definition available.

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- Definition

The maximum asset size to be able to include issued trust preferred securities as Tier 1 regulatory capital under the Basel III Capital Rules.

+ References

No definition available.

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- Definition

Maximum time to defer payment of interest on subordinate debt.

+ References

No definition available.

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- Definition

The number of statutory business trusts formed for the purpose of issuing or participating in pools of trust preferred capital securities.

+ References

No definition available.

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- Definition

Amount of borrowings as of the balance sheet date from the Federal Home Loan Bank, which are primarily used to cover shortages in the required reserve balance and liquidity shortages.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

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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.

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No definition available.

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- Definition

The calculated weighted average interest rate for Federal Home Loan Bank advances for the period ended as of the Balance Sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13(3)(a))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

Amount of advances or credit lines available from the Federal Home Loan Bank (FHLB).

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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- Definition

Carrying value as of the balance sheet date of long-term debt (with maturities initially due after one year or beyond the operating cycle if longer) identified as Junior Subordinated Notes, which have a lower priority than senior instruments, excluding current portion.

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