Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Investment Securities [Line Items]
Due within 1 year, Amortized Cost		$ 1,098
Due after 1 but within 5 years, Amortized Cost		13,989
Due after 5 but within 10 years, Amortized Cost		27,083
Due after 10 years, Amortized Cost		29,417
Debt securities gross, Amortized Cost		71,587
Amortized Cost		740,846	$ 790,082
Total debt securities, Amortized Cost		739,176
Due within 1 year, Estimated Fair Value		1,097
Due after 1 but within 5 years, Estimated Fair Value		14,020
Due after 5 but within 10 years, Estimated Fair Value		27,410
Due after 10 years, Estimated Fair Value		26,119
Debt securities gross, Estimated Fair Value		68,646
Estimated Fair Value of Available for Sale		731,358	$ 778,612
Total debt securities, Estimated Fair Value		729,688
Mortgage-Backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	[1]	667,589
Estimated Fair Value of Available for Sale	[1]	$ 661,042

[1] Mortgage Backed Securities include an amortized cost of $35.2 million and a fair value of $35.5 million for Obligations of U.S. Government agencies issued by Ginnie Mae and an amortized cost of $632.4 million and a fair value of $625.6 million for Obligations of U.S. Government-sponsored enterprises issued by Fannie Mae and Freddie Mac.