Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018
Investment Securities [Line Items]
Due within 1 year, Amortized Cost		$ 0
Due after 1 but within 5 years, Amortized Cost		35,886
Due after 5 but within 10 years, Amortized Cost		5,771
Due after 10 years, Amortized Cost		0
Debt securities gross, Amortized Cost		41,657
Amortized Cost		896,495	$ 924,054
Total debt securities, Amortized Cost		896,495
Due within 1 year, Estimated Fair Value		0
Due after 1 but within 5 years, Estimated Fair Value		36,554
Due after 5 but within 10 years, Estimated Fair Value		6,317
Due after 10 years, Estimated Fair Value		0
Debt securities gross, Estimated Fair Value		42,871
Securities available for sale		902,292	$ 909,247
Total debt securities, Estimated Fair Value		902,292
Mortgage-Backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	[1]	854,838
Securities available for sale	[1]	$ 859,421

[1] Mortgage Backed Securities include an amortized cost of $194.1 million and a fair value of $197.5 million for Obligations of U.S. Government agencies issued by Ginnie Mae and an amortized cost of $660.8 million and a fair value of $661.9 million for Obligations of U.S. Government-sponsored enterprises issued by Fannie Mae and Freddie Mac.