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Investment Securities - Additional Information (Detail)
$ / shares in Units, $ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
USD ($)
Security
Jun. 30, 2016
USD ($)
Jun. 30, 2017
USD ($)
Security
Jun. 30, 2016
USD ($)
Jul. 31, 2017
USD ($)
$ / shares
Dec. 31, 2016
USD ($)
Schedule of Available-for-sale Securities [Line Items]            
Number of available-for-sale investment securities in the less than twelve month category (security) | Security 12   12      
Number of available-for-sale investment securities in the twelve month or more category (security) | Security 7   7      
Impairment loss on investment securities $ 2,882 $ 0 $ 4,585 $ 0    
Cost basis 1,001,421   1,001,421     $ 497,869
Fair value 1,012,605   1,012,605     493,474
Pledged investment securities fair value 642,600   642,600     231,300
Equity securities            
Schedule of Available-for-sale Securities [Line Items]            
Cost basis 10,661   10,661     15,246
Fair value $ 10,661   $ 10,661     $ 15,246
Subsequent Event            
Schedule of Available-for-sale Securities [Line Items]            
Share price (in dollars per share) | $ / shares         $ 1.57  
Subsequent Event | Equity securities            
Schedule of Available-for-sale Securities [Line Items]            
Fair value         $ 6,300