Securities - Available for Sale and Held to Maturity Securities with Gross Unrealized Losses (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Fair Value	$ 20,407	$ 195,113	$ 41,673
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses	69	13,103	431
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value	128,598	4,621
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses	2,215	379
Available for Sale and Held to Maturity Securities, Total, Fair Value	149,005	199,734	41,673
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	2,284	13,482	431
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Fair Value	4,968	29,708	9,887
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses	15	1,042	55
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value	14,795
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses	145
Available for Sale and Held to Maturity Securities, Total, Fair Value	19,763	29,708	9,887
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	160	1,042	55
U.S. Government agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Fair Value	9,954	113,446	30,335
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses	22	10,322	364
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value	92,923	4,621
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses	1,909	379
Available for Sale and Held to Maturity Securities, Total, Fair Value	102,877	118,067	30,335
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	1,931	10,701	364
Mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Fair Value		44,269
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses		1,269
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value	19,436
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses	136
Available for Sale and Held to Maturity Securities, Total, Fair Value	19,436	44,269
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	136	1,269
States and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Fair Value	5,485	7,690	1,451
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses	32	470	12
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value	1,444
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses	25
Available for Sale and Held to Maturity Securities, Total, Fair Value	6,929	7,690	1,451
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	$ 57	$ 470	$ 12