Cash and Cash Equivalents and Securities - Additional Information (Detail)	3 Months Ended
Mar. 31, 2020 USD ($) Security	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Schedule of Investments [Line Items]
Accrued interest receivable, available-for-sale	$ 11,400,000
Accrued interest receivable, held-to-maturity	32,000,000.0
Debt security available for sale with a fair value pledged with collateral for public deposits	2,460,000,000		$ 2,430,000,000
Debt security available for sale with a held-to-maturity pledged with collateral for public deposits	$ 1,620,000,000		1,470,000,000
Number of debt securities owned \| Security	272
Debt securities with unrealized losses	$ 100,000		6,900,000
Number of debt securities classified as available-for-sale \| Security	2
Available for sale securities unrealized losses	$ 100,000		6,900,000
Held to maturity securities gross unrealized losses	2,800,000		700,000
Other-than-temporary impairment losses recognized in earnings	0	$ 0
Debt securities, allowance for credit loss	1,900,000
Debt securities, held-to-maturity nonaccrual	0
Unrealized (losses) gains due to change in fair value of equity securities	(1,400,000)	$ 200,000
Impairment of investments	$ 0
Federal Reserve Bank stock percentage	6.00%
Federal Reserve Bank of New York
Schedule of Investments [Line Items]
Interest-bearing deposits	$ 525,600,000		$ 219,400,000
Investment interest rate	0.10%		1.55%
Acquired shares of capital stock	$ 227,800,000		$ 203,800,000
FHLB of New York
Schedule of Investments [Line Items]
Acquired shares of capital stock	700,000		700,000
FHLB of Boston
Schedule of Investments [Line Items]
Acquired shares of capital stock	178,700,000		136,600,000
GSE mortgage-backed securities
Schedule of Investments [Line Items]
Debt securities with unrealized losses	100,000		4,100,000
Available for sale securities unrealized losses	100,000		4,100,000
Held to maturity securities gross unrealized losses	$ 0		300,000
Available for sale securities average maturity period	27 years
Debt securities, allowance for credit loss	$ 0
State and municipal
Schedule of Investments [Line Items]
Held to maturity securities gross unrealized losses	1,500,000		400,000
Debt securities, allowance for credit loss	100,000
Corporate
Schedule of Investments [Line Items]
Held to maturity securities gross unrealized losses	1,300,000		$ 0
Debt securities, allowance for credit loss	$ 1,800,000

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Debt Securities, Held-To-Maturity, Accrued Interest Receivable

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Disclosure regarding factors used to determine that the impairment of debt securities where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the average contractual maturity period of the debt securities determined to be temporarily impaired.

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Percentage of the Company's capital and surplus, forming the value of shares of Federal Reserve Bank capital stock the Company is required to purchase and hold.

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Number of debt securities owned.

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) 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 accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity).

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Amount of investment in debt security measured at amortized cost (held-to-maturity) on nonaccrual status.

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Amount of unrealized gain (loss) on 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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Federal Reserve Bank stock represents an equity interest in the Federal Reserve Bank. 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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For banks and other depository institutions (including Federal Reserve Banks, if applicable): Interest-bearing deposits in other financial institutions for relatively short periods of time including, for example, certificates of deposits, which are presented separately from cash on the balance sheet.

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Rate of interest on investment.

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The carrying amount as of the date of the latest financial statement presented of other securities classified as available for sale or held for investment which are owned but transferred to another party to serve as collateral to partially or fully secure a debt obligation, repurchase agreement or other current or potential obligation, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

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The carrying amount as of the date of the latest financial statement presented of other debt securities classified as held to maturity which are owned but transferred to another party to serve as collateral to partially or fully secure a debt obligation, repurchase agreement or other current or potential obligation, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

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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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