INVESTMENT SECURITIES - Additional Information (Details)	6 Months Ended	12 Months Ended
Jun. 30, 2018 USD ($) secuirty	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Gain (Loss) on Securities [Line Items]
Trading securities	$ 0
Securities pledged as collateral	745,400,000		$ 688,200,000
Sale of investment securities available for sale	7,012,000	$ 351,614,000
Debt securities, available-for-sale, realized gain (less than)	100,000	1,100,000
Losses from sale of available-for-sale securities (less than during the current quarter end)	0	$ 100,000
Unamortized premiums	13,600,000		14,100,000
Unaccreted discounts	1,300,000		1,300,000
Owned investment securities	1,020,000,000
Total unrealized losses on securities	$ 33,000,000
Number of securities, below investment grade \| secuirty	1
Available-for-sale securities, below investment grade, fair value	$ 800,000
Weighted average duration of MBS portfolio	5 years 4 months 17 days
Collateralized Mortgage Backed Securities
Gain (Loss) on Securities [Line Items]
OTTI on evaluation of securities	$ 0		$ 0

Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
-Paragraph 9
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Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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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.

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Amount of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in earnings and other comprehensive loss (OCI).

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Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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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.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Available-For-Sale Securities, Below Investment Grade, Fair Value

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Available For Sale Securities Unamortized Discount

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Debt Securities, Available For Sale Securities Unamortized Premium

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Number of Securities, Below Investment Grade

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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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