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

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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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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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

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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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Available-For-Sale Securities, Number Of Securities

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Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities Available for Sale (Detail) $ in Thousands	Dec. 31, 2017 USD ($)	Dec. 31, 2017 security	Dec. 31, 2017 Security	Dec. 31, 2016 USD ($) Security
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 1,511,034			$ 1,315,723
Gross Unrealized Gains	5,029			4,066
Gross Unrealized Losses	(22,158)			(22,416)
Available for sale	1,493,905			1,297,373
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	50,997			51,020
Gross Unrealized Gains	0			6
Gross Unrealized Losses	(1,355)			(1,435)
Available for sale	49,642			49,591
U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	42,384			22,815
Gross Unrealized Gains	158			232
Gross Unrealized Losses	(37)			(6)
Available for sale	42,505			23,041
Obligations of states and state agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	38,435			40,696
Gross Unrealized Gains	158			70
Gross Unrealized Losses	(374)			(424)
Available for sale	38,219			40,342
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	74,752			80,045
Gross Unrealized Gains	477			147
Gross Unrealized Losses	(564)			(767)
Available for sale	74,665			79,425
Total obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	113,187			120,741
Gross Unrealized Gains	635			217
Gross Unrealized Losses	(938)			(1,191)
Available for sale	112,884			119,767
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,239,534			1,029,827
Gross Unrealized Gains	2,423			2,061
Gross Unrealized Losses	(18,662)			(16,346)
Available for sale	1,223,295			1,015,542
Trust preferred securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	3,726			10,164
Gross Unrealized Gains	0			0
Gross Unrealized Losses	(512)			(2,155)
Available for sale	3,214			$ 8,009
Number of available for sale securities		1	1	2
Corporate and other debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	50,701			$ 60,651
Gross Unrealized Gains	623			436
Gross Unrealized Losses	(160)			(522)
Available for sale	51,164			60,565
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	10,505			20,505
Gross Unrealized Gains	1,190			1,114
Gross Unrealized Losses	(494)			(761)
Available for sale	$ 11,201			$ 20,858