Summary of Investment Securities by Estimated Maturity (Detail) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Held-to-Maturity Amortized Cost
Due in one year or less	$ 317
Due after one year through five years	1,606
Due after five years through ten years	220
Due after ten years	858
Total	3,001	17,406
Held-to-Maturity Market Value
Due in one year or less	321
Due after one year through five years	1,696
Due after five years through ten years	253
Due after ten years	956
Held-to-Maturity Market Value	3,226	18,066
Available-for-Sale Amortized Cost
Due in one year or less	22,851
Due after one year through five years	846,587
Due after five years through ten years	185,129
Due after ten years	60,302
Total	1,114,869	904,546
Available-for-Sale Market Value
Due in one year or less	23,018
Due after one year through five years	861,471
Due after five years through ten years	188,286
Due after ten years	61,339
Available-for-Sale Market Value	$ 1,134,114	$ 919,110