XML 64 R51.htm IDEA: XBRL DOCUMENT v3.8.0.1
Investment Securities Held to Maturity - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2017
USD ($)
security
Dec. 31, 2016
USD ($)
security
Dec. 31, 2015
USD ($)
Schedule of Held-to-maturity Securities [Line Items]      
Held to maturity securities at carrying value $ 409,700,000 $ 384,800,000  
Investment securities held to maturity 477,652,000 488,183,000  
Fair value 485,039,000 489,287,000  
Proceeds from the calls $ 55,720,000 $ 62,975,000 $ 37,271,000
Number of securities in an unrealized loss position | security 184 332  
Held-to-maturity Securities [Member]      
Schedule of Held-to-maturity Securities [Line Items]      
Recognized gain on calls of securities held to maturity portfolio $ 60,000 $ 15,000 8,000
Recognized loss on calls of securities held to maturity portfolio 3,000 1,000 0
Proceeds from the calls 32,900,000 45,900,000 $ 27,000,000
Mortgage-backed securities [Member]      
Schedule of Held-to-maturity Securities [Line Items]      
Investment securities held to maturity 382,000 893,000  
Fair value $ 396,000 $ 924,000