Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Mortgage Backed Securities [Member]		Jun. 30, 2012 Mortgage Backed Securities [Member]		Jun. 30, 2013 Obligations of U.S. Government Agencies [Member]	Jun. 30, 2013 US Government-sponsored Enterprises Debt Securities [Member]
Investment Securities [Line Items]
Due within one year, Amortized Cost	$ 28,081
Due after one but within five years, Amortized Cost	239,966
Due after five but within ten years, Amortized Cost	0
Due after ten years, Amortized Cost	56,229
Debt securities gross, Amortized Cost	324,276
Corporate/Mortgage-Backed Securities, Amortized cost	1,332,619	1,169,613	1,006,523	[1]	1,006,347	[1]	25,100	981,400
Debt Securities, Amortized Cost	1,330,799
Due within one year, Estimated Fair Value	28,265
Due after one but within five years, Estimated Fair Value	237,114
Due after five but within ten years, Estimated Fair Value	0
Due after ten years, Estimated Fair Value	32,853
Debt securities gross, Estimated Fair Value	298,232
Total Debt Securities, Estimated Fair Value	$ 1,304,579

[1] Mortgage Backed Securities include an amortized cost of $25.1 million and a fair value of $28.0 million for Obligations of U.S. Government agencies issued by Ginnie Mae and an amortized cost of $981.4 million and a fair value of $978.3 million for Obligations of U.S. Government-sponsored enterprises issued by Fannie Mae and Freddie Mac.