Investment Securities (Securities Held to Maturity by Contractual Maturity) (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 476,359	$ 473,684
Fair value	495,516	$ 488,331
Held-to-maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less, amortized cost	14,256
Due after one year through five years, amortized cost	50,309
Due after five years through ten years, amortized cost	238,948
Due after ten years, amortized cost	171,809
Amortized cost	475,322
Due in one year or less, fair value	14,296
Due after one year through five years, fair value	51,462
Due after five years through ten years, fair value	250,709
Due after ten years, fair value	177,970
Fair value	$ 494,437