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Investment Securities (Securities Held to Maturity by Contractual Maturity) (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost $ 481,845 $ 488,183
Fair value 490,425 $ 489,287
Held-to-maturity Securities [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Due in one year or less, amortized cost 14,723  
Due after one year through five years, amortized cost 65,170  
Due after five years through ten years, amortized cost 254,191  
Due after ten years, amortized cost 147,286  
Amortized cost 481,370  
Due in one year or less, fair value 14,755  
Due after one year through five years, fair value 66,276  
Due after five years through ten years, fair value 259,908  
Due after ten years, fair value 148,994  
Fair value $ 489,933