Gross Unrealized Losses on and Fair Value of Investment Securities With Unrealized Losses Position Aggregated by Investment Category and Length of Time that Individual Securities Have Been in Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months Fair Value	$ 102,960	$ 127,467
Less than 12 Months Unrealized Losses	798	434
12 Months or More Fair Value	7,277	8,996
12 Months or More Unrealized Losses	554	755
Total Fair Value	110,237	136,463
Total Unrealized Losses	1,352	1,189
Obligations of U.S. Government sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months Fair Value	18,052	69,852
Less than 12 Months Unrealized Losses	161	166
12 Months or More Fair Value	0	6,207
12 Months or More Unrealized Losses	0	19
Total Fair Value	18,052	76,059
Total Unrealized Losses	161	185
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months Fair Value	84,908	57,615
Less than 12 Months Unrealized Losses	637	268
12 Months or More Fair Value	7,277	2,789
12 Months or More Unrealized Losses	554	736
Total Fair Value	92,185	60,404
Total Unrealized Losses	$ 1,191	$ 1,004