Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value
Less than Twelve Months	$ 324,246	$ 101,621
More than Twelve Months	323,080	1,086,150
Total	647,326	1,187,771
Unrealized Losses
Less than Twelve Months	(1,483)	(1,207)
More than Twelve Months	(10,448)	(46,532)
Total	(11,931)	(47,739)
U.S. government agency securities
Fair Value
Less than Twelve Months	5,183	0
More than Twelve Months	0	6,074
Total	5,183	6,074
Unrealized Losses
Less than Twelve Months	(41)	0
More than Twelve Months	0	(135)
Total	(41)	(135)
Obligations of states and state agencies
Fair Value
Less than Twelve Months	11,178	16,098
More than Twelve Months	32,397	138,437
Total	43,575	154,535
Unrealized Losses
Less than Twelve Months	(55)	(266)
More than Twelve Months	(474)	(5,469)
Total	(529)	(5,735)
Municipal bonds
Fair Value
Less than Twelve Months	0	3,335
More than Twelve Months	798	60,078
Total	798	63,413
Unrealized Losses
Less than Twelve Months	0	(37)
More than Twelve Months	(10)	(1,324)
Total	(10)	(1,361)
Total obligations of states and political subdivisions
Fair Value
Less than Twelve Months	11,178	19,433
More than Twelve Months	33,195	198,515
Total	44,373	217,948
Unrealized Losses
Less than Twelve Months	(55)	(303)
More than Twelve Months	(484)	(6,793)
Total	(539)	(7,096)
Residential mortgage-backed securities
Fair Value
Less than Twelve Months	307,885	72,240
More than Twelve Months	254,915	846,671
Total	562,800	918,911
Unrealized Losses
Less than Twelve Months	(1,387)	(852)
More than Twelve Months	(3,963)	(33,516)
Total	(5,350)	(34,368)
Trust preferred securities
Fair Value
Less than Twelve Months	0	0
More than Twelve Months	29,990	30,055
Total	29,990	30,055
Unrealized Losses
Less than Twelve Months	0	0
More than Twelve Months	(5,981)	(5,923)
Total	(5,981)	(5,923)
Corporate and other debt securities
Fair Value
Less than Twelve Months	0	9,948
More than Twelve Months	4,980	4,835
Total	4,980	14,783
Unrealized Losses
Less than Twelve Months	0	(52)
More than Twelve Months	(20)	(165)
Total	$ (20)	$ (217)

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for 12 months or longer.

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Fair value of investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position.

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for less than 12 months.

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Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for less than 12 months.

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