Securities with Unrealized Losses, Aggregated by Investment Category with Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 16,836	$ 197,322
Less than 12 months Unrealized Losses	(67)	(803)
12 months or more Fair Value	4,952	4,542
12 months or more Unrealized Losses	(866)	(1,276)
Total Fair Value	21,788	201,864
Total Unrealized Losses	(933)	(2,079)
U.S. Treasury securities and U.S. Government agencies
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	500	60,547
Less than 12 months Unrealized Losses	(7)	(235)
Total Fair Value	500	60,547
Total Unrealized Losses	(7)	(235)
Private label mortgage backed and other private label mortgage-related securities
Investments, Unrealized Loss Position [Line Items]
12 months or more Fair Value	4,952	4,542
12 months or more Unrealized Losses	(866)	(1,276)
Total Fair Value	4,952	4,542
Total Unrealized Losses	(866)	(1,276)
Mortgage backed securities - residential
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	16,336	136,775
Less than 12 months Unrealized Losses	(60)	(568)
Total Fair Value	16,336	136,775
Total Unrealized Losses	$ (60)	$ (568)