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SECURITIES SOLD UNDER AGREEMENTS TO REPURCHASE - Securities sold under agreements to repurchase, Underlying Securities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Assets Sold under Agreements to Repurchase [Line Items]    
Amortized Cost of Underlying Securities $ 354,962 $ 341,557
Balance of Borrowings 300,000 300,000
Approximate Fair Value of Underlying Securities $ 350,123 $ 339,390
Weighted-average interest rates on repurchase agreements 1.92% 2.35%
Interest Receivable $ 57,172 $ 45,453
Repurchase Agreements [Member]    
Assets Sold under Agreements to Repurchase [Line Items]    
Interest Receivable 1,060 1,400
US Treasury Securities And Obligations Of Other US Government Sponsored Agencies [Member]    
Assets Sold under Agreements to Repurchase [Line Items]    
Amortized Cost of Underlying Securities 132,637 126,205
Balance of Borrowings 127,801 123,175
Approximate Fair Value of Underlying Securities $ 131,271 $ 125,417
US Treasury Securities And Obligations Of Other US Government Sponsored Agencies [Member] | Repurchase Agreements [Member]    
Assets Sold under Agreements to Repurchase [Line Items]    
Weighted-average interest rates on repurchase agreements 1.39% 1.30%
Mortgage Backed Securities [Member]    
Assets Sold under Agreements to Repurchase [Line Items]    
Amortized Cost of Underlying Securities $ 222,325 $ 215,352
Balance of Borrowings 172,199 176,825
Approximate Fair Value of Underlying Securities $ 218,852 $ 213,973
Mortgage Backed Securities [Member] | Repurchase Agreements [Member]    
Assets Sold under Agreements to Repurchase [Line Items]    
Weighted-average interest rates on repurchase agreements 2.29% 2.20%