Securities - Additional Information (Detail)	6 Months Ended
Jun. 30, 2018 USD ($) Holdings	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Holdings
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Maximum percentage of equity of one issuer	10.00%
Number of holdings greater than specified percentage of equity \| Holdings	0		0
Equities securities	$ 13,494,000		$ 13,457,000
Securities with aggregate fair values	1,600,000,000		1,400,000,000
Net unrealized losses on available-for-sale debt securities included in AOCI	36,000,000		13,300,000
Sales of available-for-sale debt securities	81,500,000	$ 7,800,000
Impairment loss relating to securities	0
Federal home loan bank stock, Total	59,300,000		$ 45,900,000
Grantor Trusts [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Equities securities	$ 8,300,000

Accumulated unrealized gain (loss), after tax, on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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The carrying amount, as of the date of the latest financial statement presented, of other assets which are owned but transferred to serve as collateral for the payment of the related debt obligation, primarily a secured borrowing or repurchase agreement, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

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Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Schedule Of Available For Sale Securities And Held To Maturity [Line Items]

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