Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Investment Securities [Line Items]
Due within 1 year, Amortized Cost	$ 3,000
Due after 1 but within 5 years, Amortized Cost	266,181
Due after 5 but within 10 years, Amortized Cost	21,849
Due after 10 years, Amortized Cost	53,817
Debt securities gross, Amortized Cost	344,847
Amortized Cost	1,317,318		1,350,466
Total debt securities, Amortized Cost	1,315,898
Due within 1 year, Estimated Fair Value	2,998
Due after 1 but within 5 years, Estimated Fair Value	264,879
Due after 5 but within 10 years, Estimated Fair Value	22,104
Due after 10 years, Estimated Fair Value	41,527
Debt securities gross, Estimated Fair Value	331,508
Estimated Fair Value of Available for Sale	1,309,819		1,318,365
Total debt securities, Estimated Fair Value	1,308,399
Mortgage-Backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	971,051	[1]
Estimated Fair Value of Available for Sale	$ 976,891	[1]

[1] Mortgage Backed Securities include an amortized cost of $23.3 million and a fair value of $25.9 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 $947.7 million and a fair value of $951.0 million.