Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities Available for Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	$ 1,558,787	$ 1,796,404
Gross Unrealized Gains	13,596	1,995
Gross Unrealized Losses	(5,582)	(48,855)
Fair Value	1,566,801	1,749,544
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	50,952	50,975
Gross Unrealized Gains	12	0
Gross Unrealized Losses	(21)	(1,669)
Fair Value	50,943	49,306
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	28,982	36,844
Gross Unrealized Gains	280	71
Gross Unrealized Losses	(19)	(638)
Fair Value	29,243	36,277
Obligations of states and state agencies
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	78,116	100,777
Gross Unrealized Gains	540	18
Gross Unrealized Losses	(83)	(3,682)
Fair Value	78,573	97,113
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	90,662	101,207
Gross Unrealized Gains	902	209
Gross Unrealized Losses	(86)	(1,437)
Fair Value	91,478	99,979
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	168,778	201,984
Gross Unrealized Gains	1,442	227
Gross Unrealized Losses	(169)	(5,119)
Fair Value	170,051	197,092
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	1,248,814	1,469,059
Gross Unrealized Gains	11,234	1,484
Gross Unrealized Losses	(5,262)	(40,761)
Fair Value	1,254,786	1,429,782
Corporate and other debt securities
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	61,261	37,542
Gross Unrealized Gains	628	213
Gross Unrealized Losses	(111)	(668)
Fair Value	$ 61,778	$ 37,087

Amount, before tax, of unrealized gain 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, 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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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Amount 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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