Securities - Summaries of Fair Values and Unrealized Losses of Securities, Segregated by Length of Impairment (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014 Security	Dec. 31, 2013 Security	Dec. 31, 2012
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year	19	54
Number of available for sale securities, Greater than 1 year	28	14
Number of available for sale securities	47	68
Securities Available For Sale, Less than 12 Months, Fair Value	$ 309,919	$ 734,257
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(1,559)	(26,101)
Securities Available For Sale, 12 Months or More, Fair Value	332,215	48,144
Securities Available For Sale, 12 Months or More, Unrealized Losses	(10,362)	(13,194)
Securities Available For Sale, Fair Value, Total	642,134	782,401
Securities Available For Sale, Unrealized Losses, Total	(11,921)	(39,295)
Number of held to maturity securities	36	80
Securities Held To Maturity, Less than 12 Months, Fair Value	131,586	669,220
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(675)	(24,379)
Securities Held To Maturity, Greater than 12 Months, Fair Value	269,653	15,674
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(7,567)	(711)
Securities Held To Maturity, Fair Value, Total	401,239	684,894
Securities Held To Maturity, Unrealized Losses, Total	(8,242)	(25,090)	(135)
Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities	12	78
Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities	24	2
U.S. Government-Sponsored Entities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year	7	17
Number of available for sale securities, Greater than 1 year	7
Number of available for sale securities	14	17
Securities Available For Sale, Less than 12 Months, Fair Value	89,986	232,962
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(275)	(5,904)
Securities Available For Sale, 12 Months or More, Fair Value	99,326
Securities Available For Sale, 12 Months or More, Unrealized Losses	(1,664)
Securities Available For Sale, Fair Value, Total	189,312	232,962
Securities Available For Sale, Unrealized Losses, Total	(1,939)	(5,904)
Number of held to maturity securities	4	3
Securities Held To Maturity, Less than 12 Months, Fair Value	24,989	24,513
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(40)	(530)
Securities Held To Maturity, Greater than 12 Months, Fair Value	29,516	14,378
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(484)	(621)
Securities Held To Maturity, Fair Value, Total	54,505	38,891
Securities Held To Maturity, Unrealized Losses, Total	(524)	(1,151)	(99)
U.S. Government-Sponsored Entities [Member] \| Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities	2	2
U.S. Government-Sponsored Entities [Member] \| Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities	2	1
Agency Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year	2	9
Number of available for sale securities	2	9
Securities Available For Sale, Less than 12 Months, Fair Value	45,145	108,284
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(35)	(1,303)
Securities Available For Sale, Fair Value, Total	45,145	108,284
Securities Available For Sale, Unrealized Losses, Total	(35)	(1,303)
Number of held to maturity securities	5	25
Securities Held To Maturity, Less than 12 Months, Fair Value	1,099	308,864
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(1)	(5,942)
Securities Held To Maturity, Greater than 12 Months, Fair Value	45,042	1,296
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(345)	(90)
Securities Held To Maturity, Fair Value, Total	46,141	310,160
Securities Held To Maturity, Unrealized Losses, Total	(346)	(6,032)	(1)
Agency Mortgage-Backed Securities [Member] \| Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities	1	24
Agency Mortgage-Backed Securities [Member] \| Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities	4	1
Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year	9	26
Number of available for sale securities, Greater than 1 year	16	2
Number of available for sale securities	25	28
Securities Available For Sale, Less than 12 Months, Fair Value	166,908	389,989
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(1,238)	(18,644)
Securities Available For Sale, 12 Months or More, Fair Value	225,700	34,229
Securities Available For Sale, 12 Months or More, Unrealized Losses	(7,809)	(2,150)
Securities Available For Sale, Fair Value, Total	392,608	424,218
Securities Available For Sale, Unrealized Losses, Total	(9,047)	(20,794)
Number of held to maturity securities	22	21
Securities Held To Maturity, Less than 12 Months, Fair Value	104,071	301,312
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(630)	(15,844)
Securities Held To Maturity, Greater than 12 Months, Fair Value	189,642
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(6,699)
Securities Held To Maturity, Fair Value, Total	293,713	301,312
Securities Held To Maturity, Unrealized Losses, Total	(7,329)	(15,844)
Agency Collateralized Mortgage Obligations [Member] \| Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities	8	21
Agency Collateralized Mortgage Obligations [Member] \| Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities	14
Commercial Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year	1
Number of available for sale securities	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)
Number of held to maturity securities		1
Securities Held To Maturity, Less than 12 Months, Fair Value		984
Securities Held To Maturity, Less than 12 Months, Unrealized Losses		(37)
Securities Held To Maturity, Fair Value, Total		984
Securities Held To Maturity, Unrealized Losses, Total		(37)
Commercial Mortgage-Backed Securities [Member] \| Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities		1
States of the U.S. and Political Subdivisions [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year		2
Number of available for sale securities, Greater than 1 year	1
Number of available for sale securities	1	2
Securities Available For Sale, Less than 12 Months, Fair Value		3,022
Securities Available For Sale, Less than 12 Months, Unrealized Losses		(250)
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	3,022
Securities Available For Sale, Unrealized Losses, Total	(32)	(250)
Number of held to maturity securities	5	27
Securities Held To Maturity, Less than 12 Months, Fair Value	1,427	31,537
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(4)	(2,022)
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	6,880	31,537
Securities Held To Maturity, Unrealized Losses, Total	(43)	(2,022)
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	1	27
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	4
Other Securities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Greater than 1 year	4	4
Number of available for sale securities	4	4
Securities Available For Sale, 12 Months or More, Fair Value	6,030	5,950
Securities Available For Sale, 12 Months or More, Unrealized Losses	(857)	(929)
Securities Available For Sale, Fair Value, Total	6,030	5,950
Securities Available For Sale, Unrealized Losses, Total	(857)	(929)
Collateralized Debt Obligations [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Greater than 1 year		8
Number of available for sale securities		8
Securities Available For Sale, 12 Months or More, Fair Value		7,965
Securities Available For Sale, 12 Months or More, Unrealized Losses		(10,115)
Securities Available For Sale, Fair Value, Total		7,965
Securities Available For Sale, Unrealized Losses, Total		(10,115)
Securities Held To Maturity, Unrealized Losses, Total			(35)
Non-Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities		3
Securities Held To Maturity, Less than 12 Months, Fair Value		2,010
Securities Held To Maturity, Less than 12 Months, Unrealized Losses		(4)
Securities Held To Maturity, Fair Value, Total		2,010
Securities Held To Maturity, Unrealized Losses, Total		$ (4)
Non-Agency Collateralized Mortgage Obligations [Member] \| Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities		3

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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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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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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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