Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Investment Securities [Line Items]
Due within one year, Estimated Fair Value	$ 7,007
Due after one but within five years, Estimated Fair Value	235,961
Due after five but within ten years, Estimated Fair Value	0
Due after ten years, Estimated Fair Value	34,195
Debt securities gross, Estimated Fair Value	277,163
Corporate/Mortgage-Backed Securities, Estimated fair value	1,130,822		1,142,776
Total Debt Securities, Estimated Fair Value	1,128,962
Due within one year, Amortized Cost	7,001
Due after one but within five years, Amortized Cost	235,077
Due after five but within ten years, Amortized Cost	0
Due after ten years, Amortized Cost	63,467
Debt securities gross, Amortized Cost	305,545
Corporate/Mortgage-Backed Securities, Amortized cost	1,123,728		1,140,212
Total Debt Securities, Amortized Cost	1,121,868
Mortgage Backed Securities [Member]
Investment Securities [Line Items]
Corporate/Mortgage-Backed Securities, Estimated fair value	851,799	[1]
Corporate/Mortgage-Backed Securities, Amortized cost	$ 816,323	[1]

[1] Mortgage Backed Securities include an amortized cost of $29.2 million and a fair value of $33.4 million for Obligations of U.S. Government agencies issued by Ginnie Mae and Obligations of U.S. Government-sponsored enterprises issued by Fannie Mae and Freddie Mac which had an amortized cost of $787.1 million and a fair value of $818.4 million.