Securities - Amortized Cost of Securities and their Estimated Fair Values (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
FHLB stock	$ 4,428	$ 4,428
FHLB stock	4,428	4,428
Amortized Cost	182,379	184,940
Gross Unrealized Gains	1,121	1,414
Gross Unrealized Losses	(3,288)	(1,563)
Estimated Fair Value	180,212	184,791
Estimated Fair Value, Less than 12 months	101,849	87,323
Unrealized Losses, Less than 12 months	(2,167)	(772)
Estimated Fair Value, 12 months or longer	32,218	32,325
Unrealized Losses, 12 months or longer	(1,121)	(791)
Estimated Fair Value	134,067	119,648
Unrealized Losses	(3,288)	(1,563)
U.S. Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	81,263	84,210
Gross Unrealized Gains	241	536
Gross Unrealized Losses	(1,358)	(653)
Estimated Fair Value	80,146	84,093
U.S. Agency Securities [Member] | Temporarily Impaired Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 months	50,671	41,501
Unrealized Losses, Less than 12 months	(1,046)	(431)
Estimated Fair Value, 12 months or longer	10,343	9,846
Unrealized Losses, 12 months or longer	(312)	(222)
Estimated Fair Value	61,014	51,347
Unrealized Losses	(1,358)	(653)
Taxable Municipals Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,277	1,279
Gross Unrealized Gains	1	5
Gross Unrealized Losses	(5)	(1)
Estimated Fair Value	1,273	1,283
Taxable Municipals Bonds [Member] | Temporarily Impaired Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 months	515	521
Unrealized Losses, Less than 12 months	(5)	(1)
Estimated Fair Value	515	521
Unrealized Losses	(5)	(1)
Tax Free Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26,412	26,412
Gross Unrealized Gains	441	637
Gross Unrealized Losses	(185)	(83)
Estimated Fair Value	26,668	26,966
Tax Free Municipal Bonds [Member] | Temporarily Impaired Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 months	6,810	4,860
Unrealized Losses, Less than 12 months	(140)	(51)
Estimated Fair Value, 12 months or longer	900	913
Unrealized Losses, 12 months or longer	(45)	(32)
Estimated Fair Value	7,710	5,773
Unrealized Losses	(185)	(83)
Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	71,772	71,389
Gross Unrealized Gains	113	201
Gross Unrealized Losses	(1,740)	(826)
Estimated Fair Value	70,145	70,764
Mortgage-Backed Securities [Member] | Temporarily Impaired Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 months	43,853	40,441
Unrealized Losses, Less than 12 months	(976)	(289)
Estimated Fair Value, 12 months or longer	20,975	21,566
Unrealized Losses, 12 months or longer	(764)	(537)
Estimated Fair Value	64,828	62,007
Unrealized Losses	(1,740)	(826)
Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,655	1,650
Gross Unrealized Gains	325	35
Estimated Fair Value	$ 1,980	$ 1,685