Investment Securities - Summary of Amortized Cost and Approximate Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 472,300		$ 497,869
Gross Unrealized Gains	2,639		1,319
Gross Unrealized Losses	(3,568)		(5,714)
Investment securities available for sale, fair value	471,371		493,474
Agency-guaranteed residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	186,221		233,002
Gross Unrealized Gains	36		918
Gross Unrealized Losses	(2,799)		(2,657)
Investment securities available for sale, fair value	183,458		231,263
Agency-guaranteed commercial real estate mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	238,809		204,689
Gross Unrealized Gains	432		0
Gross Unrealized Losses	(769)		(2,872)
Investment securities available for sale, fair value	238,472		201,817
Corporate notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	44,959		44,932
Gross Unrealized Gains	1,130		401
Gross Unrealized Losses	0		(185)
Investment securities available for sale, fair value	46,089		45,148
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,311	$ 2,300	15,246
Gross Unrealized Gains	1,041		0
Gross Unrealized Losses	0		0
Investment securities available for sale, fair value	$ 3,352		$ 15,246

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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-Publisher FASB
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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