| **DESCRIPTION OF BUSINESS, BASIS OF PRESENTATION AND ACCOUNTING POLICIES (General) (Details) - USD ($) $ in Thousands** |             | 3 Months Ended | 9 Months Ended |             |             |
|-------------------------------------------------------------------------------------------------------------------------|-------------|----------------|----------------|-------------|-------------|
||
| **Noncontrolling Interest [Line Items]**                                                                                |        |           |           |        |        |   |   |   |
| Income tax (benefit)/provision                                                                                          |        | $ (53,343)     |           | $ 112,927   | $ (272,856) | $ 384,467 |   |   |
| Total assets                                                                                                            |        | 145,739,595    |           | 147,185,414 | 145,739,595 | 147,185,414 |   | $ 149,499,477 |
| Total liabilities                                                                                                       |        | 124,934,624    |           |        | 124,934,624 |   |   | 125,100,647 |
| Stockholder's equity                                                                                                    |        | 19,532,351     |           |        | 19,532,351  |   |   | 22,021,460 |
| Net (loss)/income including NCI                                                                                         |        | 874,536        |           | 270,436     | (1,144,182) | 916,292 |   |   |
| Santander BanCorp                                                                                                       |        |           |           |        |        |   |   |   |
| **Noncontrolling Interest [Line Items]**                                                                                |        |           |           |        |        |   |   |   |
| Total assets                                                                                                            |        |           |           |        |        |   | $ 5,500,000 | 6,000,000 |
| Total liabilities                                                                                                       |        |           |           |        |        |   | 4,300,000 | 4,800,000 |
| Stockholder's equity                                                                                                    |        |           |           |        |        |   | $ 1,200,000 | $ 1,200,000 |
| Net (loss)/income including NCI                                                                                         |        | 14,300         |           | $ 26,400    | 33,100      | $ 66,600 |   |   |
| Disposal group, not discontinued operation, loans transferred                                                           |        |           |           |        | 160,000     |   |   |   |
| Disposal group, not discontinued operation, real estate owned, transferred                                              |        |           |           |        | $ 30,000    |   |   |   |
| Disposal Group, Not Discontinued Operations | Santander BanCorp                                                         |        |           |           |        |        |   |   |   |
| **Noncontrolling Interest [Line Items]**                                                                                |        |           |           |        |        |   |   |   |
| Sale of stock, consideration received on transaction                                                                    | $ 1,280,000 |           |           |        |        |   |   |   |
| Proceeds from divestiture of interest in consolidated subsidiaries                                                      |        | 62,000         |           |        |        |   |   |   |
| Income tax (benefit)/provision                                                                                          |        | 12,000         | $ 39,000       |        |        |   |   |   |
| Gain (loss) on disposition of business                                                                                  |        | 50,000         |           |        |        |   |   |   |
| Reclassification from accumulated other comprehensive income, current period, before tax                                |        | 23,600         |           |        |        |   |   |   |
| Reclassification from accumulated other comprehensive income, current period, net of tax                                |        | 14,800         |           |        |        |   |   |   |
| Disposal group, not discontinued operation, transaction expenses                                                        |        | $ 10,000       |           |        |        |   |   |   |
| SC                                                                                                                      |        |           |           |        |        |   |   |   |
| **Noncontrolling Interest [Line Items]**                                                                                |        |           |           |        |        |   |   |   |
| Ownership percentage by parent                                                                                          |        | 80.20%         |           |        | 80.20%      |   |   |   |
| Percentage owned by noncontrolling shareholders                                                                         |        | 19.80%         |           |        | 19.80%      |   |   |   |

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
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Amount of gain (loss) from sale and disposal of integrated set of activities and assets capable of being conducted and managed for purpose of providing return in form of dividend, lower cost, or other economic benefit to investor, owner, member and participant.
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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.
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Sum of the carrying amounts as of the balance sheet date of all liabilities that are recognized. Liabilities are probable future sacrifices of economic benefits arising from present obligations of an entity to transfer assets or provide services to other entities in the future.
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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.
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The cash inflow associated with the deconsolidation of a previously consolidated subsidiary or the sale of investment in consolidated subsidiaries (generally greater than 50 percent).
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The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.
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Amount before tax of reclassification adjustments of other comprehensive income (loss).
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Amount after tax of reclassification adjustments of other comprehensive income (loss).
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| - Definition
Cash received on stock transaction after deduction of issuance costs.
+ References
No definition available.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.
+ References
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