Securities - Summaries of Fair Values and Unrealized Losses of Securities, Segregated by Length of Impairment (Detail) $ in Thousands	Jun. 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	23	19
Number of available for sale securities, Greater than 1 year | Security	28	28
Number of available for sale securities | Security	51	47
Securities Available For Sale, Less than 12 Months, Fair Value	$ 377,323	$ 309,919
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(1,938)	(1,559)
Securities Available For Sale, 12 Months or More, Fair Value	300,467	332,215
Securities Available For Sale, 12 Months or More, Unrealized Losses	(8,757)	(10,362)
Securities Available For Sale, Fair Value, Total	677,790	642,134
Securities Available For Sale, Unrealized Losses, Total	$ (10,695)	$ (11,921)
Number of held to maturity securities | Security	83	36
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 341,503	$ 131,586
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(4,646)	(675)
Securities Held To Maturity, Greater than 12 Months, Fair Value	192,548	269,653
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(6,093)	(7,567)
Securities Held To Maturity, Fair Value, Total	534,051	401,239
Securities Held To Maturity, Unrealized Losses, Total	$ (10,739)	$ (8,242)
Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	67	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	5	7
Number of available for sale securities, Greater than 1 year | Security	5	7
Number of available for sale securities | Security	10	14
Securities Available For Sale, Less than 12 Months, Fair Value	$ 84,445	$ 89,986
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(571)	(275)
Securities Available For Sale, 12 Months or More, Fair Value	55,389	99,326
Securities Available For Sale, 12 Months or More, Unrealized Losses	(602)	(1,664)
Securities Available For Sale, Fair Value, Total	139,834	189,312
Securities Available For Sale, Unrealized Losses, Total	$ (1,173)	$ (1,939)
Number of held to maturity securities | Security	4	4
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 44,772	$ 24,989
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(227)	(40)
Securities Held To Maturity, Greater than 12 Months, Fair Value	14,849	29,516
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(152)	(484)
Securities Held To Maturity, Fair Value, Total	59,621	54,505
Securities Held To Maturity, Unrealized Losses, Total	$ (379)	$ (524)
U.S. Government-Sponsored Entities [Member] | Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	3	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	11	2
Number of available for sale securities | Security	11	2
Securities Available For Sale, Less than 12 Months, Fair Value	$ 186,261	$ 45,145
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(801)	(35)
Securities Available For Sale, Fair Value, Total	186,261	45,145
Securities Available For Sale, Unrealized Losses, Total	$ (801)	$ (35)
Number of held to maturity securities | Security	11	5
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 115,966	$ 1,099
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(974)	(1)
Securities Held To Maturity, Greater than 12 Months, Fair Value	1,127	45,042
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(32)	(345)
Securities Held To Maturity, Fair Value, Total	117,093	46,141
Securities Held To Maturity, Unrealized Losses, Total	$ (1,006)	$ (346)
Agency Mortgage-Backed Securities [Member] | Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	10	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	6	9
Number of available for sale securities, Greater than 1 year | Security	18	16
Number of available for sale securities | Security	24	25
Securities Available For Sale, Less than 12 Months, Fair Value	$ 104,538	$ 166,908
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(563)	(1,238)
Securities Available For Sale, 12 Months or More, Fair Value	237,775	225,700
Securities Available For Sale, 12 Months or More, Unrealized Losses	(7,376)	(7,809)
Securities Available For Sale, Fair Value, Total	342,313	392,608
Securities Available For Sale, Unrealized Losses, Total	$ (7,939)	$ (9,047)
Number of held to maturity securities | Security	21	22
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 95,239	$ 104,071
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(884)	(630)
Securities Held To Maturity, Greater than 12 Months, Fair Value	176,572	189,642
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(5,909)	(6,699)
Securities Held To Maturity, Fair Value, Total	271,811	293,713
Securities Held To Maturity, Unrealized Losses, Total	$ (6,793)	$ (7,329)
Agency Collateralized Mortgage Obligations [Member] | Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	7	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, Less than 1 year | Security	1
Number of available for sale securities, Greater than 1 year | Security	1	1
Number of available for sale securities | Security	2	1
Securities Available For Sale, Less than 12 Months, Fair Value	$ 2,079
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(3)
Securities Available For Sale, 12 Months or More, Fair Value	1,153	$ 1,159
Securities Available For Sale, 12 Months or More, Unrealized Losses	(37)	(32)
Securities Available For Sale, Fair Value, Total	3,232	1,159
Securities Available For Sale, Unrealized Losses, Total	$ (40)	$ (32)
Number of held to maturity securities | Security	45	5
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 83,795	$ 1,427
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(2,558)	(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	83,795	6,880
Securities Held To Maturity, Unrealized Losses, Total	$ (2,558)	$ (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	45	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	4	4
Number of available for sale securities | Security	4	4
Securities Available For Sale, 12 Months or More, Fair Value	$ 6,150	$ 6,030
Securities Available For Sale, 12 Months or More, Unrealized Losses	(742)	(857)
Securities Available For Sale, Fair Value, Total	6,150	6,030
Securities Available For Sale, Unrealized Losses, Total	$ (742)	$ (857)
Non-Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	2
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 1,731
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(3)
Securities Held To Maturity, Fair Value, Total	1,731
Securities Held To Maturity, Unrealized Losses, Total	$ (3)
Non-Agency Collateralized Mortgage Obligations [Member] | Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	2