Investment Securities (Securities Held to Maturity by Contractual Maturity) (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 472,934	$ 473,684
Fair value	491,349	$ 488,331
Available-for-sale Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less, amortized cost	11,538
Due after one year through five years, amortized cost	52,697
Due after five years through ten years, amortized cost	221,145
Due after ten years, amortized cost	186,151
Amortized cost	471,531
Due in one year or less, fair value	11,589
Due after one year through five years, fair value	53,797
Due after five years through ten years, fair value	231,977
Due after ten years, fair value	192,528
Fair value	$ 489,891