Investment Securities - Summary of Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value of Investment Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	$ 398,375		$ 384,422
Available for sale, Gross Unrealized Gains	2,109		7,067
Available for sale, Gross Unrealized Losses	(5,169)		(1,079)
Available for sale, Fair Value	395,315	$ 395,315	390,410
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	120,437		115,535
Available for sale, Gross Unrealized Gains	244		1,891
Available for sale, Gross Unrealized Losses	(1,591)		(173)
Available for sale, Fair Value	119,090		117,253
Freddie Mac [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	95,669		84,486
Available for sale, Gross Unrealized Gains	73		1,369
Available for sale, Gross Unrealized Losses	(1,136)		(85)
Available for sale, Fair Value	94,606		85,770
Governmental National Mortgage Association [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	14,610		16,091
Available for sale, Gross Unrealized Gains	34		76
Available for sale, Gross Unrealized Losses	(218)		(28)
Available for sale, Fair Value	14,426		16,139
Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	230,716		216,112
Available for sale, Gross Unrealized Gains	351		3,336
Available for sale, Gross Unrealized Losses	(2,945)		(286)
Available for sale, Fair Value	228,122		219,162
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	68,995		71,323
Available for sale, Gross Unrealized Gains	1,314		2,432
Available for sale, Gross Unrealized Losses	(974)		(65)
Available for sale, Fair Value	69,335		73,690
U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	23,344		25,669
Available for sale, Gross Unrealized Gains	119		272
Available for sale, Fair Value	23,463		25,941
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	45,315		38,331
Available for sale, Gross Unrealized Gains	203		599
Available for sale, Gross Unrealized Losses	(738)		(512)
Available for sale, Fair Value	44,780		38,418
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	29,980		32,962
Available for sale, Gross Unrealized Gains	122		428
Available for sale, Gross Unrealized Losses	(512)		(216)
Available for sale, Fair Value	29,590		33,174
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	398,350		384,397
Available for sale, Gross Unrealized Gains	2,109		7,067
Available for sale, Gross Unrealized Losses	(5,169)		(1,079)
Available for sale, Fair Value	395,290		390,385
Equity Securities-Financial Services [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	25		25
Available for sale, Fair Value	$ 25		$ 25