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Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Investment Securities [Line Items]
Due within 1 year, Amortized Cost		$ 4,000
Due after 1 but within 5 years, Amortized Cost		17,256
Due after 5 but within 10 years, Amortized Cost		35,319
Due after 10 years, Amortized Cost		1,929
Debt securities gross, Amortized Cost		58,504
Amortized Cost		924,054	$ 740,846
Total debt securities, Amortized Cost		924,054
Due within 1 year, Estimated Fair Value		3,993
Due after 1 but within 5 years, Estimated Fair Value		17,045
Due after 5 but within 10 years, Estimated Fair Value		35,448
Due after 10 years, Estimated Fair Value		2,250
Debt securities gross, Estimated Fair Value		58,736
Securities available for sale		909,247	$ 731,358
Total debt securities, Estimated Fair Value		909,247
Mortgage-Backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	[1]	865,550
Securities available for sale	[1]	$ 850,511
[1] Mortgage Backed Securities include an amortized cost of $178.6 million and a fair value of $177.2 million for Obligations of U.S. Government agencies issued by Ginnie Mae and an amortized cost of $686.9 million and a fair value of $673.4 million for Obligations of U.S. Government-sponsored enterprises issued by Fannie Mae and Freddie Mac.

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