Investment Securities (Narrative) (Detail) (USD $)	1 Months Ended	9 Months Ended			9 Months Ended			12 Months Ended
	Sep. 30, 2012 Y	Sep. 30, 2012 Y	Jun. 30, 2012 Y	Dec. 31, 2011	Sep. 30, 2012 Collateralized Mortgage Backed Securities [Member] Security	Sep. 30, 2011 Collateralized Mortgage Backed Securities [Member]	Sep. 30, 2012 S A S C O R M 1 Two Thousand And Two Class B Securities [Member]	Dec. 31, 2011 S A S C O R M 1 Two Thousand And Two Class O Securities [Member]	Sep. 30, 2012 S A S C O R M 1 Two Thousand And Two Class O Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities					164
Securities amortized cost	$ 881,469,000	$ 881,469,000		$ 840,584,000	$ 828,200,000
Securities pledged as collateral	431,200,000	431,200,000
FHLB pledged investment securities	0	0
Proceeds from sale of investment securities		616,300,000				216,100,000
Gain from sale of investment securities		17,700,000				3,000,000
Weighted average duration of the mortgage-backed securities	4.2	4.2	4.5
Unamortized premiums		22,600,000
Unaccreted discounts		216,000
Owned Investment Securities	17,100,000	17,100,000
Total unrealized losses on securities	189,000	189,000
Trading securities par value							12,500,000	2,500,000
Interest paid in kind	1,500,000
Investments purchase percentage								100.00%
Trading securities fair value									$ 6,100,000