| **INVESTMENT SECURITIES (Securities Pledged as Collateral) (Details) - USD ($) $ in Millions** | Mar. 31, 2016 | Dec. 31, 2015 |
|------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financial Instruments Owned and Pledged as Collateral [Line Items]**                         |          |          |
| Securities pledged as collateral, fair value                                                   | $ 7,500.0     | $ 3,500.0     |
| Public fund deposits                                                                           |          |          |
| **Financial Instruments Owned and Pledged as Collateral [Line Items]**                         |          |          |
| Securities pledged as collateral, fair value                                                   | 4,300.0       | 2,900.0       |
| Repurchase agreements, hedging activities and recourse on loan sales                           |          |          |
| **Financial Instruments Owned and Pledged as Collateral [Line Items]**                         |          |          |
| Securities pledged as collateral, fair value                                                   | 2,700.0       | 117.6         |
| Overnight customer deposits                                                                    |          |          |
| **Financial Instruments Owned and Pledged as Collateral [Line Items]**                         |          |          |
| Securities pledged as collateral, fair value                                                   | $ 459.0       | $ 395.8       |

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| - Definition
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.
+ References
No definition available.
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| - Definition
Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 50 -Paragraph 1A -Subparagraph (b)(1)(i) -URI http://asc.fasb.org/extlink&oid=55761124&loc=SL6224234-111729Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6467456&loc=d3e113888-111728
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