Amortized Cost and Fair Value of Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Securities Available-for-Sale:
Due in one year or less	$ 2,372
Due after one year through five years	14,817
Due after five years through ten years	26,967
Due after ten years	29,052
Totals	592,015	500,128
Securities Available-for-Sale, Fair Value
Due in one year or less	2,385
Due after one year through five years	15,273
Due after five years through ten years	29,072
Due after ten years	31,748
Totals	612,396	516,844
Securities Held-to-Maturity, Carrying Amount
Due in one year or less	0
Due after one year through five years	346
Due after five years through ten years	0
Due after ten years	0
Totals	346	690
Securities Held-to-Maturity, Fair Value
Due in one year or less	0
Due after one year through five years	351
Due after five years through ten years	0
Due after ten years	0
Fair Value	351	697
Mortgage-backed Securities - Residential [Member]
Securities Available-for-Sale:
Due in one year or less	0
Due after one year through five years	0
Due after five years through ten years	0
Due after ten years	0
Available-for-Sale, amortized cost	518,123
Totals	518,123	431,495
Securities Available-for-Sale, Fair Value
Due in one year or less	0
Due after one year through five years	0
Due after five years through ten years	0
Due after ten years	0
Available-for-Sale, Fair Value	533,212
Totals	533,212	443,934
Equity Securities [Member]
Securities Available-for-Sale:
Due in one year or less	0
Due after one year through five years	0
Due after five years through ten years	0
Due after ten years	0
Available-for-Sale, amortized cost	684
Totals	684	684
Securities Available-for-Sale, Fair Value
Due in one year or less	0
Due after one year through five years	0
Due after five years through ten years	0
Due after ten years	0
Available-for-Sale, Fair Value	706
Totals	$ 706	$ 684