Investment Securities - Investment Securities (Parenthetical) (Detail)	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Contractual Maturities of Investment Securities [Abstract]
Federal statutory rate	21.00%	21.00%
Weighted-average maturity of available-for-sale investment securities	4 years	5 years 4 months 24 days
Weighted-average yield of available-for-sale investment securities	2.56%	2.57%
Weighted-average maturity of held-to-maturity investment securities	3 years 9 months 18 days	5 years 2 months 12 days
Weighted-average yield of held-to-maturity investment securities	2.42%	2.46%

Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I)
-URI http://asc.fasb.org/extlink&oid=116825942&loc=d3e330036-122817

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration