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Impairment of Investment Securities - Additional Information (Detail)		12 Months Ended
		Dec. 31, 2018 USD ($) Security	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Number of securities in an unrealized loss positions | Security		132
Corporate/Mortgage-Backed Securities, Amortized cost		$ 924,054,000	$ 740,846,000
Securities available for sale		909,247,000	731,358,000
Other than-temporary impairment charge	[1]	9,164,000	0	$ 0
Other investments		30,456,000	29,837,000
Equity Securities, FV-NI		1,670,000
Equity Securities, FV-NI, Gain (Loss)		0
Threshold Percentage of Value Decline in Equity Securities to be considered Other than Temporary Impairment			0	0
Debt Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Corporate/Mortgage-Backed Securities, Amortized cost		924,054,000	739,176,000
Securities available for sale		909,247,000	729,688,000
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Debt and Equity Securities, FV-NI [Line Items]
Corporate/Mortgage-Backed Securities, Amortized cost		0	27,499,000
Securities available for sale		0	23,646,000
Other than-temporary impairment charge		$ 0	0	$ 0
Obligations of U.S. Government-Sponsored Enterprises [Member]
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses		82.00%
Corporate Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Corporate/Mortgage-Backed Securities, Amortized cost		$ 20,912,000	15,907,000
Securities available for sale		$ 21,012,000	16,493,000
Debt Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Securities available for sale			729,688,000
US States and Political Subdivisions Debt Securities [Member] | Other Government - Sponsored Enterprises [Member]
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses		16.00%
Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage - Backed Securities - Residential [Member]
Debt and Equity Securities, FV-NI [Line Items]
Number of securities in an unrealized loss positions		0
Corporate/Mortgage-Backed Securities, Amortized cost		$ 686,906,000	632,422,000
Securities available for sale		673,361,000	625,555,000
Obligations of U.S. Government-Sponsored Enterprises [Member] | Other Government - Sponsored Enterprises [Member]
Debt and Equity Securities, FV-NI [Line Items]
Corporate/Mortgage-Backed Securities, Amortized cost		10,000,000	1,098,000
Securities available for sale		$ 10,012,000	$ 1,097,000
[1] Represents the increase in cash flows recognized either as principal payments or interest income during the period.

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Percentage Of Total Unrealized Losses

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

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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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Amount of unrealized and realized 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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