Gross Unrealized Losses and Fair Value by Length of Time for Held-To-Maturity Securities (Detail) (Held-to-Maturity Securities, USD $) In Thousands, unless otherwise specified	Jun. 30, 2013 Securities	Dec. 31, 2012 Securities
Gain (Loss) on Investments [Line Items]
Number of securities	111	14
Fair Value	$ 149,593	$ 26,829
Less than 12 months	7,918	152
More than 12 months	0	0
Total Unrealized Losses	7,918	152
U.S. government agencies
Gain (Loss) on Investments [Line Items]
Number of securities	8	1
Fair Value	61,432	9,961
Less than 12 months	3,070	29
More than 12 months	0	0
Total Unrealized Losses	3,070	29
State and municipal
Gain (Loss) on Investments [Line Items]
Number of securities	103	13
Fair Value	88,162	16,868
Less than 12 months	4,848	123
More than 12 months	0	0
Total Unrealized Losses	$ 4,848	$ 123