Investment Securities - Additional Information (Detail) (USD $)	3 Months Ended				3 Months Ended			12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Municipal bonds [Member]	Mar. 31, 2013 Mortgage-backed securities ("MBS") [Member] Security Y	Mar. 31, 2013 U.S. Government and government sponsored enterprises ("GSE") [Member] Mortgage-backed securities ("MBS") [Member]	Mar. 31, 2013 SASCO RM-1 2002 [Member]	Dec. 31, 2011 SASCO 2002-RM1 Class O Securities [Member]	Mar. 31, 2013 SASCO 2002-RM1 Class O Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities					143
Securities amortized cost	$ 817,243,000		$ 886,031,000			$ 747,100,000
Weighted average duration of MBS portfolio					5.4
Securities pledged as collateral	443,600,000
Proceeds from sale of investment securities	139,200,000	84,800,000
Gain from sale of investment securities	1,600,000	2,000,000
Unamortized premiums	22,600,000
Unaccreted discounts	138,000
Owned investment securities	238,800,000
Total unrealized losses on securities	2,600,000
Municipal bonds				17,600,000
Trading securities, par value							12,600,000	2,500,000
Investments purchase percentage								100.00%
Trading securities, fair value									$ 6,100,000