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INVESTMENT SECURITIES - Aggregate amortized cost and market value of the securities by issuers (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule Of Available-For-Sale And Held-To-Maturity Securities [Line Items]    
Amortized cost $ 1,903,878 $ 1,908,557
Fair Value 1,891,016 1,881,920
Federal Home Loan Mortgage Corporation Certificates And Obligations F H L M C [Member]    
Schedule Of Available-For-Sale And Held-To-Maturity Securities [Line Items]    
Amortized cost 375,719 399,955
Fair Value 370,855 394,249
Government National Mortgage Association Certificates And Obligations G N M A [Member]    
Schedule Of Available-For-Sale And Held-To-Maturity Securities [Line Items]    
Amortized cost 250,140 254,495
Fair Value 257,192 265,615
Federal National Mortgage Association Certificates And Obligations F N M A [Member]    
Schedule Of Available-For-Sale And Held-To-Maturity Securities [Line Items]    
Amortized cost 801,198 849,584
Fair Value 796,726 843,818
FHLB [Member]    
Schedule Of Available-For-Sale And Held-To-Maturity Securities [Line Items]    
Amortized cost 299,949 231,666
Fair Value $ 296,767 $ 229,792