Investment Securities - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Securities pledged as collateral	$ 562,500,000	$ 457,000,000
Proceeds from sale of investment securities	201,800,000	192,800,000
Gains from sale of Available-for-sale securities	2,400,000	1,500,000
Unamortized premiums	18,000,000
Unaccreted discounts	400,000
Owned investment securities	668,900,000
Total unrealized losses on securities	$ 16,600,000
Weighted average duration of MBS portfolio	5 years 4 months 24 days
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Losses from sale of Available-for-sale securities		100,000
Mortgage-Backed Securities ("MBS") [Member]
Schedule of Available-for-sale Securities [Line Items]
OTTI on evaluation of securities	$ 0	$ 0

This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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This element represents the excess of the cost (face amount, notional amount) of an investment (security, contract) over its fair value which deficiency has not been recognized in earnings of the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=50485924&loc=SL6090603-123010

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The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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-SubTopic 10
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The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Investing Activities
-URI http://asc.fasb.org/extlink&oid=6516133

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The carrying value as of the balance sheet date of the assets received as collateral against securities loaned to other broker-dealers. Borrowers of securities generally are required to provide collateral to the lenders of securities, commonly cash but sometimes other securities or standby letters of credit, with a value slightly higher than that of the securities borrowed. In instances where the entity is permitted to sell or re-pledge these securities, the entity reports the fair value of the collateral received and the related obligation to return the collateral as a liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 10
-Section 05
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=64836671&loc=d3e93645-111703

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Available For Sale Securities Unamortized Discount

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Available For Sale Securities Unamortized Premium

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized as held-to-maturity or available-for-sale.

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Weighted Average Remaining Expected Life Of Residential Mortgage Backed Securities Available For Sale One

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