Debt and Equity Securities - Gross Unrealized Losses and Fair Value of Company's Investments (Detail) $ in Thousands	Sep. 30, 2015 USD ($) Security	Dec. 31, 2014 USD ($) Security
Held To Maturity Debt Securities [Line Items]
Held to Maturity Securities, Fair Value, Less than 12 Month	$ 7,025	$ 0
Held to Maturity Securities, Unrealized losses, Less than 12 Month	$ 41	$ 0
Held to Maturity Securities, Number of securities included, Less than 12 Month | Security	16	0
Held to Maturity Securities, Fair Value, 12 Month or More	$ 3,046	$ 7,251
Held to Maturity Securities, Unrealized losses, 12 Month or More	$ 123	$ 188
Held to Maturity Securities, Number of securities included, 12 Month or More | Security	4	10
Held to Maturity Securities, Total Fair Value	$ 10,071	$ 7,251
Held to Maturity Securities, Total Unrealized Losses	164	188
Available-for-Sale Securities, Fair Value, Less than 12 Month	74,956	119,449
Available-for-Sale Securities, Unrealized Losses, Less than 12 Month	$ 455	$ 635
Available-for-Sale Securities, Number of Securities Included, Less than 12 Month | Security	48	59
Available-for-Sale Securities, Fair Value, 12 Month or More	$ 44,987	$ 101,121
Available-for-Sale Securities, Unrealized Loss, 12 Month or More	$ 443	$ 1,521
Available-for-Sale Securities, Number of securities included, 12 Month or More | Security	29	50
Available-for-Sale Securities, Total Fair Value	$ 119,943	$ 220,570
Available-for-Sale Securities, Total Unrealized Losses	898	2,156
US Government-Sponsored Enterprises (GSEs) and Agency-Backed Securities [Member]
Held To Maturity Debt Securities [Line Items]
Available-for-Sale Securities, Fair Value, Less than 12 Month	0	34,753
Available-for-Sale Securities, Unrealized Losses, Less than 12 Month	$ 0	$ 143
Available-for-Sale Securities, Number of Securities Included, Less than 12 Month | Security	0	10
Available-for-Sale Securities, Fair Value, 12 Month or More	$ 34,967	$ 74,250
Available-for-Sale Securities, Unrealized Loss, 12 Month or More	$ 276	$ 1,186
Available-for-Sale Securities, Number of securities included, 12 Month or More | Security	11	24
Available-for-Sale Securities, Total Fair Value	$ 34,967	$ 109,003
Available-for-Sale Securities, Total Unrealized Losses	276	1,329
Government- Sponsored Enterprises (GSEs) Residential Mortgage Backed [Member]
Held To Maturity Debt Securities [Line Items]
Held to Maturity Securities, Fair Value, Less than 12 Month	1,576	0
Held to Maturity Securities, Unrealized losses, Less than 12 Month	$ 14	$ 0
Held to Maturity Securities, Number of securities included, Less than 12 Month | Security	1	0
Held to Maturity Securities, Fair Value, 12 Month or More	$ 2,748	$ 4,674
Held to Maturity Securities, Unrealized losses, 12 Month or More	$ 121	$ 147
Held to Maturity Securities, Number of securities included, 12 Month or More | Security	3	4
Held to Maturity Securities, Total Fair Value	$ 4,324	$ 4,674
Held to Maturity Securities, Total Unrealized Losses	135	147
Available-for-Sale Securities, Fair Value, Less than 12 Month	62,178	66,504
Available-for-Sale Securities, Unrealized Losses, Less than 12 Month	$ 386	$ 279
Available-for-Sale Securities, Number of Securities Included, Less than 12 Month | Security	32	36
Available-for-Sale Securities, Fair Value, 12 Month or More	$ 4,198	$ 22,172
Available-for-Sale Securities, Unrealized Loss, 12 Month or More	$ 85	$ 185
Available-for-Sale Securities, Number of securities included, 12 Month or More | Security	8	13
Available-for-Sale Securities, Total Fair Value	$ 66,376	$ 88,676
Available-for-Sale Securities, Total Unrealized Losses	471	464
SBAP [Member]
Held To Maturity Debt Securities [Line Items]
Available-for-Sale Securities, Fair Value, Less than 12 Month	8,701	16,114
Available-for-Sale Securities, Unrealized Losses, Less than 12 Month	$ 36	$ 205
Available-for-Sale Securities, Number of Securities Included, Less than 12 Month | Security	5	9
Available-for-Sale Securities, Fair Value, 12 Month or More	$ 2,513	$ 0
Available-for-Sale Securities, Unrealized Loss, 12 Month or More	$ 1	$ 0
Available-for-Sale Securities, Number of securities included, 12 Month or More | Security	1	0
Available-for-Sale Securities, Total Fair Value	$ 11,214	$ 16,114
Available-for-Sale Securities, Total Unrealized Losses	37	205
Obligations of States and Political Subdivisions [Member]
Held To Maturity Debt Securities [Line Items]
Held to Maturity Securities, Fair Value, Less than 12 Month	5,449	0
Held to Maturity Securities, Unrealized losses, Less than 12 Month	$ 27	$ 0
Held to Maturity Securities, Number of securities included, Less than 12 Month | Security	15	0
Held to Maturity Securities, Fair Value, 12 Month or More	$ 298	$ 2,577
Held to Maturity Securities, Unrealized losses, 12 Month or More	$ 2	$ 41
Held to Maturity Securities, Number of securities included, 12 Month or More | Security	1	6
Held to Maturity Securities, Total Fair Value	$ 5,747	$ 2,577
Held to Maturity Securities, Total Unrealized Losses	29	41
Available-for-Sale Securities, Fair Value, Less than 12 Month	4,077	2,078
Available-for-Sale Securities, Unrealized Losses, Less than 12 Month	$ 33	$ 8
Available-for-Sale Securities, Number of Securities Included, Less than 12 Month | Security	11	4
Available-for-Sale Securities, Fair Value, 12 Month or More	$ 3,309	$ 4,699
Available-for-Sale Securities, Unrealized Loss, 12 Month or More	$ 81	$ 150
Available-for-Sale Securities, Number of securities included, 12 Month or More | Security	9	13
Available-for-Sale Securities, Total Fair Value	$ 7,386	$ 6,777
Available-for-Sale Securities, Total Unrealized Losses	$ 114	$ 158