Securities - Summaries of Fair Values and Unrealized Losses of Securities, Segregated by Length of Impairment (Detail) $ in Thousands	Sep. 30, 2015 USD ($) Security	Dec. 31, 2014 USD ($) Security
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security	8	19
Number of available for sale securities, Greater than 1 year \| Security	24	28
Number of available for sale securities \| Security	32	47
Securities Available For Sale, Less than 12 Months, Fair Value	$ 142,054	$ 309,919
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(382)	(1,559)
Securities Available For Sale, 12 Months or More, Fair Value	270,357	332,215
Securities Available For Sale, 12 Months or More, Unrealized Losses	(5,367)	(10,362)
Securities Available For Sale, Fair Value, Total	412,411	642,134
Securities Available For Sale, Unrealized Losses, Total	$ (5,749)	$ (11,921)
Number of held to maturity securities \| Security	43	36
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 93,053	$ 131,586
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(1,038)	(675)
Securities Held To Maturity, Greater than 12 Months, Fair Value	186,204	269,653
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(4,064)	(7,567)
Securities Held To Maturity, Fair Value, Total	279,257	401,239
Securities Held To Maturity, Unrealized Losses, Total	$ (5,102)	$ (8,242)
Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities \| Security	27	12
Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities \| Security	16	24
U.S. Government-Sponsored Entities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security	3	7
Number of available for sale securities, Greater than 1 year \| Security	3	7
Number of available for sale securities \| Security	6	14
Securities Available For Sale, Less than 12 Months, Fair Value	$ 59,823	$ 89,986
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(177)	(275)
Securities Available For Sale, 12 Months or More, Fair Value	37,867	99,326
Securities Available For Sale, 12 Months or More, Unrealized Losses	(128)	(1,664)
Securities Available For Sale, Fair Value, Total	97,690	189,312
Securities Available For Sale, Unrealized Losses, Total	$ (305)	$ (1,939)
Number of held to maturity securities \| Security	1	4
Securities Held To Maturity, Less than 12 Months, Fair Value		$ 24,989
Securities Held To Maturity, Less than 12 Months, Unrealized Losses		(40)
Securities Held To Maturity, Greater than 12 Months, Fair Value	$ 14,921	29,516
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(79)	(484)
Securities Held To Maturity, Fair Value, Total	14,921	54,505
Securities Held To Maturity, Unrealized Losses, Total	$ (79)	$ (524)
U.S. Government-Sponsored Entities [Member] \| Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities \| Security		2
U.S. Government-Sponsored Entities [Member] \| Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities \| Security	1	2
Agency Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security	1	2
Number of available for sale securities \| Security	1	2
Securities Available For Sale, Less than 12 Months, Fair Value	$ 18,591	$ 45,145
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(82)	(35)
Securities Available For Sale, Fair Value, Total	18,591	45,145
Securities Available For Sale, Unrealized Losses, Total	$ (82)	$ (35)
Number of held to maturity securities \| Security	1	5
Securities Held To Maturity, Less than 12 Months, Fair Value		$ 1,099
Securities Held To Maturity, Less than 12 Months, Unrealized Losses		(1)
Securities Held To Maturity, Greater than 12 Months, Fair Value	$ 924	45,042
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(16)	(345)
Securities Held To Maturity, Fair Value, Total	924	46,141
Securities Held To Maturity, Unrealized Losses, Total	$ (16)	$ (346)
Agency Mortgage-Backed Securities [Member] \| Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities \| Security		1
Agency Mortgage-Backed Securities [Member] \| Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities \| Security	1	4
Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security	4	9
Number of available for sale securities, Greater than 1 year \| Security	18	16
Number of available for sale securities \| Security	22	25
Securities Available For Sale, Less than 12 Months, Fair Value	$ 63,640	$ 166,908
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(123)	(1,238)
Securities Available For Sale, 12 Months or More, Fair Value	228,010	225,700
Securities Available For Sale, 12 Months or More, Unrealized Losses	(4,825)	(7,809)
Securities Available For Sale, Fair Value, Total	291,650	392,608
Securities Available For Sale, Unrealized Losses, Total	$ (4,948)	$ (9,047)
Number of held to maturity securities \| Security	17	22
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 46,120	$ 104,071
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(212)	(630)
Securities Held To Maturity, Greater than 12 Months, Fair Value	170,359	189,642
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(3,969)	(6,699)
Securities Held To Maturity, Fair Value, Total	216,479	293,713
Securities Held To Maturity, Unrealized Losses, Total	$ (4,181)	$ (7,329)
Agency Collateralized Mortgage Obligations [Member] \| Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities \| Security	3	8
Agency Collateralized Mortgage Obligations [Member] \| Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities \| Security	14	14
Commercial Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security		1
Number of available for sale securities \| Security		1
Securities Available For Sale, Less than 12 Months, Fair Value		$ 7,880
Securities Available For Sale, Less than 12 Months, Unrealized Losses		(11)
Securities Available For Sale, Fair Value, Total		7,880
Securities Available For Sale, Unrealized Losses, Total		$ (11)
States of the U.S. and Political Subdivisions [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Greater than 1 year \| Security		1
Number of available for sale securities \| Security		1
Securities Available For Sale, 12 Months or More, Fair Value		$ 1,159
Securities Available For Sale, 12 Months or More, Unrealized Losses		(32)
Securities Available For Sale, Fair Value, Total		1,159
Securities Available For Sale, Unrealized Losses, Total		$ (32)
Number of held to maturity securities \| Security	24	5
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 46,933	$ 1,427
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(826)	(4)
Securities Held To Maturity, Greater than 12 Months, Fair Value		5,453
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses		(39)
Securities Held To Maturity, Fair Value, Total	46,933	6,880
Securities Held To Maturity, Unrealized Losses, Total	$ (826)	$ (43)
States of the U.S. and Political Subdivisions [Member] \| Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities \| Security	24	1
States of the U.S. and Political Subdivisions [Member] \| Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities \| Security		4
Other Securities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Greater than 1 year \| Security	3	4
Number of available for sale securities \| Security	3	4
Securities Available For Sale, 12 Months or More, Fair Value	$ 4,480	$ 6,030
Securities Available For Sale, 12 Months or More, Unrealized Losses	(414)	(857)
Securities Available For Sale, Fair Value, Total	4,480	6,030
Securities Available For Sale, Unrealized Losses, Total	$ (414)	$ (857)

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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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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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity that have been in a continuous loss position for twelve months or longer.

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Disclosure regarding factors used to determine that the impairment of securities categorized as held-to-maturity where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the number of investment positions in the held-to-maturity investments determined to be temporarily impaired.

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