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Short-Term Borrowings - Additional Information (Detail) - USD ($)
1 Months Ended 6 Months Ended
May 31, 2016
Jun. 30, 2016
Dec. 31, 2015
Jun. 30, 2015
Short-term Debt [Line Items]        
Gross outstanding balance of repurchase agreements collateralized by securities percentage   111.00%    
Revolving line of credit   $ 567,659,000 $ 628,499,000 $ 530,377,000
Revolving Credit Facility [Member]        
Short-term Debt [Line Items]        
Revolving line of credit $ 75,000,000 $ 0    
Revolving loan term 364 days      
Debt instrument variable rate 2.00%      
Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]        
Short-term Debt [Line Items]        
Debt instrument description   One-month LIBOR plus 200 basis points