Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Mortgage Backed Securities [Member]		Sep. 30, 2012 Mortgage Backed Securities [Member]		Sep. 30, 2013 US Government Agencies Residential Mortgage-backed Securities [Member]	Dec. 31, 2012 US Government Agencies Residential Mortgage-backed Securities [Member]	Sep. 30, 2013 Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investment Securities [Line Items]
Due within one year, Amortized Cost	$ 28,081
Due after one but within five years, Amortized Cost	239,969
Due after five but within ten years, Amortized Cost	0
Due after ten years, Amortized Cost	55,521
Debt securities gross, Amortized Cost	323,571
Corporate/Mortgage-Backed Securities, Amortized cost	1,321,239	1,169,613	996,248	[1]	990,800	[1]	23,787	27,883	972,500
Debt Securities, Amortized Cost	1,319,819
Due within one year, Estimated Fair Value	28,215
Due after one but within five years, Estimated Fair Value	238,170
Due after five but within ten years, Estimated Fair Value	0
Due after ten years, Estimated Fair Value	32,458
Debt securities gross, Estimated Fair Value	298,843
Total Debt Securities, Estimated Fair Value	$ 1,289,643

[1] Mortgage Backed Securities include an amortized cost of $23.8 million and a fair value of $26.5 million for Obligations of U.S. Government agencies issued by Ginnie Mae and an amortized cost of $972.5 million and a fair value of $964.3 million for Obligations of U.S. Government-sponsored enterprises issued by Fannie Mae and Freddie Mac.