Securities - Securities Held to Maturity in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Held To Maturity Securities [Line Items]
Fair value, Less than 12 months	$ 608	$ 64,756
Unrealized loss, Less than 12 months	(7)	(1,329)
Fair value, 12 months or more	72,792	26,426
Unrealized loss, 12 months or more	(1,925)	(1,287)
Total fair value	73,400	91,182
Total unrealized loss	(1,932)	(2,616)
Mortgage-Backed GSE Securities: Residential [Member]
Schedule Of Held To Maturity Securities [Line Items]
Fair value, Less than 12 months		57,340
Unrealized loss, Less than 12 months		(1,243)
Fair value, 12 months or more	72,792	26,426
Unrealized loss, 12 months or more	(1,925)	(1,287)
Total fair value	72,792	83,766
Total unrealized loss	(1,925)	(2,530)
States of the U.S. and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Fair value, Less than 12 months	608	7,416
Unrealized loss, Less than 12 months	(7)	(86)
Total fair value	608	7,416
Total unrealized loss	$ (7)	$ (86)