Investment Securities (Investment Securities Available for Sale by Contractual Maturity) (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 1,047,690	$ 1,041,247
Securities available for sale, at fair value	1,048,119	$ 1,040,386
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, amortized cost	51,483
Due after one year through five years, amortized cost	9,452
Due after five years through ten years, amortized cost	18,875
Due after ten years, amortized cost	0
Amortized cost	79,810
Due in one year or less, fair value	51,466
Due after one year through five years, fair value	9,516
Due after five years through ten years, fair value	19,261
Due after ten years , fair value	0
Securities available for sale, at fair value	$ 80,243