Investments - Available for Sale and Held to Maturity Securities with Gross Unrealized Losses (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 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	$ 190,380	$ 41,673
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses	8,606	431
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses
Available for Sale and Held to Maturity Securities, Total, Fair Value	190,380	41,673
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	8,606	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	30,082	9,887
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses	652	55
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses
Available for Sale and Held to Maturity Securities, Total, Fair Value	30,082	9,887
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	652	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	117,322	30,335
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses	6,417	364
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses
Available for Sale and Held to Maturity Securities, Total, Fair Value	117,322	30,335
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	6,417	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	35,002
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses	1,058
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses
Available for Sale and Held to Maturity Securities, Total, Fair Value	35,002
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	1,058
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	7,974	1,451
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses	479	12
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses
Available for Sale and Held to Maturity Securities, Total, Fair Value	7,974	1,451
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	$ 479	$ 12