Fair Value Measurements - Investment Securities Characteristics (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Fixed Rate [Member]
Mortgage-Backed and Agency debentures characteristics [Line Items]
Weighted Average Coupon, percentage	4.74%	4.92%
Weighted Average Yield, percentage	3.83%	4.00%
Adjustable Rate [Member]
Mortgage-Backed and Agency debentures characteristics [Line Items]
Weighted Average Coupon, percentage	4.12%	4.28%
Weighted Average Yield, percentage	3.22%	3.04%
Weighted Average Lifetime Cap, percentage	10.08%	10.16%
Weighted Average Term to Next Adjustment, months	P42M	P39M