| **INVESTMENT SECURITIES (Tables)**                                    | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
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| **Investments, Debt and Equity Securities [Abstract]**                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| Summary of Available-for-sale Securities                              | The following tables present the amortized cost, gross unrealized gains and losses and approximate fair values of securities available-for-sale at the dates indicated: March 31, 2016 AmortizedCost GrossUnrealizedGains GrossUnrealizedLoss FairValue (in thousands)U.S. Treasury securities$349,274 $468 $— $349,742Corporate debt securities1,417,264 15,972 (3,646) 1,429,590Asset-backed securities ("ABS")2,038,606 9,621 (1,648) 2,046,579Equity securities10,964 5 (282) 10,687State and municipal securities21,278 349 — 21,627Mortgage-backed securities ("MBS"):       U.S. government agencies - Residential5,871,610 51,967 (18,121) 5,905,456U.S. government agencies - Commercial1,094,751 16,994 (1,992) 1,109,753FHLMC and FNMA - Residential debt securities (1)9,498,678 80,842 (60,101) 9,519,419FHLMC and FNMA - Commercial debt securities137,804 2,749 (327) 140,226Non-agency securities37 — — 37Total investment securities available-for-sale$20,440,266 $178,967 $(86,117) $20,533,116(1) Federal Home Loan Mortgage Corporation ("FHLMC") and Federal National Mortgage Association ("FNMA") December 31, 2015 AmortizedCost GrossUnrealizedGains GrossUnrealizedLoss FairValue (in thousands)U.S. Treasury securities$3,192,411 $1,658 $(5,681) $3,188,388Corporate debt securities1,476,801 10,021 (11,248) 1,475,574Asset-backed securities1,763,178 7,826 (1,494) 1,769,510Equity securities10,894 1 (408) 10,487State and municipal securities748,696 19,616 (432) 767,880Mortgage-backed securities:       U.S. government agencies - Residential4,033,041 10,225 (36,513) 4,006,753U.S. government agencies - Commercial1,006,161 3,347 (7,153) 1,002,355FHLMC and FNMA - Residential debt securities8,636,745 10,556 (153,128) 8,494,173FHLMC and FNMA - Commercial debt securities138,094 723 (2,487) 136,330Non-agency securities45 — — 45Total investment securities available-for-sale$21,006,066 $63,973 $(218,544) $20,851,495                                                                                                                                                                                                                                                                                                                                                                                                                           |
| Investments Classified by Contractual Maturity Date                   | Contractual maturities of the Company’s debt securities available-for-sale at March 31, 2016 are as follows: AmortizedCost FairValue (in thousands)Due within one year $545,999 $547,001Due after 1 year but within 5 years3,073,546 3,094,514Due after 5 years but within 10 years318,364 320,688Due after 10 years16,491,393 16,560,226Total$20,429,302 $20,522,429                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
| Gross Unrealized Loss and Fair Value of Securities Available-for-Sale | The following tables present the aggregate amount of unrealized losses as of March 31, 2016 and December 31, 2015 on securities in the Company’s available-for-sale investment portfolio classified according to the amount of time that those securities have been in a continuous loss position: March 31, 2016 Less than 12 months 12 months or longer Total Fair Value UnrealizedLosses Fair Value UnrealizedLosses Fair Value UnrealizedLosses (in thousands)Corporate debt securities$261,839 $(1,900) $174,222 $(1,746) $436,061 $(3,646)Asset-backed securities260,657 (1,037) 67,243 (611) 327,900 (1,648)Equity securities293 (2) 9,900 (280) 10,193 (282)Mortgage-backed securities:           U.S. government agencies - Residential762,829 (9,966) 733,921 (8,155) 1,496,750 (18,121)U.S. government agencies - Commercial6,109 (28) 113,681 (1,964) 119,790 (1,992)FHLMC and FNMA - Residential debt securities814,228 (3,550) 2,075,785 (56,551) 2,890,013 (60,101)FHLMC and FNMA - Commercial debt securities23,855 (327) — — 23,855 (327)Total$2,129,810 $(16,810) $3,174,752 $(69,307) $5,304,562 $(86,117) NOTE 3. INVESTMENT SECURITIES (continued) December 31, 2015 Less than 12 months 12 months or longer Total Fair Value UnrealizedLosses Fair Value UnrealizedLosses Fair Value UnrealizedLosses (in thousands)U.S. Treasury securities$2,243,343 $(5,681) $— $— $2,243,343 $(5,681)Corporate debt securities775,366 (5,269) 152,486 (5,979) 927,852 (11,248)Asset-backed securities300,869 (1,083) 35,126 (411) 335,995 (1,494)Equity securities596 (7) 9,748 (401) 10,344 (408)State and municipal securities15,665 (119) 26,024 (313) 41,689 (432)Mortgage-backed securities:           U.S. government agencies - Residential1,670,150 (11,164) 954,916 (25,349) 2,625,066 (36,513)U.S. government agencies - Commercial367,706 (3,382) 114,038 (3,771) 481,744 (7,153)FHLMC and FNMA - Residential debt securities4,650,327 (38,013) 2,127,962 (115,115) 6,778,289 (153,128)FHLMC and FNMA - Commercial debt securities115,347 (2,487) — — 115,347 (2,487)Total$10,139,369 $(67,205) $3,420,300 $(151,339) $13,559,669 $(218,544) |
| Gains (Losses) and Proceeds on Sales of Securities                    | Proceeds from sales of investment securities and the realized gross gains and losses from those sales are as follows: Three-Month PeriodEnded March 31,  2016 2015 (in thousands)Proceeds from the sales of available-for-sale securities$3,435,844 $1,010,515    Gross realized gains$26,707 $10,782Gross realized losses(276) (1,225)OTTI(10) —    Net realized gains$26,421 $9,557                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |

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Tabular disclosure of fair value of investments in debt and equity securities in an unrealized loss position categorized neither as held-to-maturity nor trading securities.
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Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.
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Tabular disclosure of maturities of an entity's investments as well as any other information pertinent to the investments.
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Tabular disclosure of realized gains and losses on investments reported in the statement of income.
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