| **INVESTMENT SECURITIES**                              | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                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| **Investments, Debt and Equity Securities [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               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| INVESTMENT SECURITIES                                  | NOTE 4. INVESTMENT SECURITIESInvestments Available-for-saleInvestment Securities Summary - Available-for-saleThe following tables present the amortized cost, gross unrealized gains and losses and approximate fair values of securities available-for-sale at the dates indicated: September 30, 2015 AmortizedCost GrossUnrealizedGains GrossUnrealizedLoss FairValue (in thousands)U.S. Treasury securities$3,191,564 $18,299 $— $3,209,863Corporate debt securities1,554,216 17,224 (8,341) 1,563,099Asset-backed securities ("ABS")1,938,581 7,111 (1,041) 1,944,651Equity securities10,826 3 (301) 10,528State and municipal securities750,946 12,524 (2,148) 761,322Mortgage-backed securities:       U.S. government agencies - Residential4,077,361 31,836 (16,724) 4,092,473U.S. government agencies - Commercial986,989 10,283 (5,109) 992,163FHLMC and FNMA - Residential debt securities (1)8,379,831 46,998 (89,154) 8,337,675FHLMC and FNMA - Commercial debt securities138,383 1,402 (427) 139,358Non-agency securities52 — — 52Total investment securities available-for-sale$21,028,749 $145,680 $(123,245) $21,051,184(1) Federal Home Loan Mortgage Corporation ("FHLMC") and Federal National Mortgage Association ("FNMA") December 31, 2014 AmortizedCost GrossUnrealizedGains GrossUnrealizedLoss FairValue (in thousands)U.S. Treasury securities$1,692,838 $2,985 $(56) $1,695,767Corporate debt securities2,159,681 29,630 (6,910) 2,182,401Asset-backed securities2,707,207 17,787 (4,591) 2,720,403Equity securities10,619 3 (279) 10,343State and municipal securities1,790,776 35,071 (2,385) 1,823,462Mortgage-backed securities:       U.S. government agencies - Residential2,151,111 1,626 (33,811) 2,118,926U.S government agencies - Commercial472,611 183 (7,186) 465,608FHLMC and FNMA - Residential debt securities4,971,045 12,817 (129,990) 4,853,872FHLMC and FNMA - Commercial debt securities23,929 157 (171) 23,915Non-agency securities12,842 539 — 13,381Total investment securities available-for-sale$15,992,659 $100,798 $(185,379) $15,908,078The Company continuously evaluates its investment strategies in light of changes in the regulatory and market environments that could have an impact on capital and liquidity. Based on this evaluation, it is reasonably possible that the Company may elect to pursue other strategies relative to its investment securities portfolio. During the second quarter of 2015, SHUSA sold $234.5 million of qualifying residential loans to FHLMC in return for $243.9 million of mortgage-backed securities ("MBS") issued by FHLMC resulting in a net realized gain on sale of $9.8 million which is included in Mortgage banking income, net line of the Company's Condensed Consolidated Statement of Operations.NOTE 4. INVESTMENT SECURITIES (continued)As of September 30, 2015 and December 31, 2014, the Company had investment securities available-for-sale with an estimated fair value of $4.1 billion and $3.5 billion, respectively, pledged as collateral, which was made up of the following: $3.5 billion and $2.6 billion were pledged to secure public fund deposits; $122.6 million and $301.6 million, respectively, were pledged at various independent parties ("Brokers") to secure repurchase agreements, support hedging relationships, and for recourse on loan sales; and $450.6 million and $560.6 million, respectively, were pledged to secure the Bank's customer overnight sweep product.At September 30, 2015 and December 31, 2014, the Company had $64.5 million and $67.3 million, respectively, of accrued interest related to investment securities which is included in the Accrued Interest Receivable line of the Company's Condensed Consolidated Balance Sheet.The Company's state and municipal bond portfolio primarily consists of general obligation bonds of states, cities, counties and school districts. The portfolio had a weighted average underlying credit risk rating of AA+ as of September 30, 2015. The largest geographic concentrations of state and local municipal bonds are in Washington, Massachusetts, and Connecticut, which represented 21.4%, 14.5%, and 12.2%, respectively, of the total portfolio. No other state comprised more than 10% of the total portfolio. Contractual Maturity of Debt SecuritiesContractual maturities of the Company’s debt securities available-for-sale at September 30, 2015 are as follows: AmortizedCost FairValue (in thousands)Due within one year$676,336 $674,564Due after 1 year but within 5 years5,847,161 5,883,134Due after 5 years but within 10 years361,297 363,618Due after 10 years14,133,129 14,119,340Total$21,017,923 $21,040,656Actual maturities may differ from contractual maturities when there is a right to call or prepay obligations with or without call or prepayment penalties.Gross Unrealized Loss and Fair Value of Securities Available-for-SaleThe following tables present the aggregate amount of unrealized losses as of September 30, 2015 and December 31, 2014 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: September 30, 2015 Less than 12 months 12 months or longer Total Fair Value UnrealizedLosses Fair Value UnrealizedLosses Fair Value UnrealizedLosses (in thousands)Corporate debt securities$500,998 $(6,427) $136,932 $(1,914) $637,930 $(8,341)Asset-backed securities143,150 (1,041) — — 143,150 (1,041)Equity securities376 (2) 9,850 (299) 10,226 (301)State and municipal securities203,170 (1,217) 25,430 (931) 228,600 (2,148)Mortgage-backed securities:           U.S. government agencies - Residential400,513 (735) 972,042 (15,989) 1,372,555 (16,724)U.S government agencies - Commercial216,806 (2,551) 116,893 (2,558) 333,699 (5,109)FHLMC and FNMA - Residential1,219,050 (4,080) 2,245,972 (85,074) 3,465,022 (89,154)FHLMC and FNMA - Commercial42,195 (427) — — 42,195 (427)Total$2,726,258 $(16,480) $3,507,119 $(106,765) $6,233,377 $(123,245)NOTE 4. INVESTMENT SECURITIES (continued) December 31, 2014 Less than 12 months 12 months or longer Total Fair Value UnrealizedLosses Fair Value UnrealizedLosses Fair Value UnrealizedLosses (in thousands)US Treasury securities$298,914 $(56) $— $— $298,914 $(56)Corporate debt securities538,108 (3,262) 214,852 (3,648) 752,960 (6,910)Asset-backed securities632,936 (1,437) 424,333 (3,154) 1,057,269 (4,591)Equity securities55 — 9,879 (279) 9,934 (279)State and municipal securities45,128 (90) 192,091 (2,295) 237,219 (2,385)Mortgage-backed securities:           U.S. government agencies - Residential415,731 (2,693) 1,348,908 (31,118) 1,764,639 (33,811)U.S. government agencies - Commercial281,258 (2,459) 136,269 (4,727) 417,527 (7,186)FHLMC and FNMA - Residential debt securities399,176 (2,019) 2,607,695 (127,971) 3,006,871 (129,990)FHLMC and FNMA - Commercial debt securities11,269 (171) — — 11,269 (171)Total$2,622,575 $(12,187) $4,934,027 $(173,192) $7,556,602 $(185,379)Other-Than-Temporary Impairment ("OTTI")Management evaluates all investment products in an unrealized loss position for OTTI on at least a quarterly basis. Individual securities are further assessed for OTTI as deemed necessary. An unrealized loss exists when the current fair value of an individual security is less than its amortized cost basis. The OTTI assessment is a subjective process requiring the use of judgments and assumptions. During the securities-level assessments, consideration is given to (1) the intent not to sell and probability that the Company will not be required to sell the security before recovery of its cost basis to allow for any anticipated recovery in fair value, (2) the financial condition and near-term prospects of the issuer, as well as Company news and current events, and (3) the ability to collect the future expected cash flows. Key assumptions utilized to forecast expected cash flows may include loss severity, expected cumulative loss percentage, cumulative loss percentage to date, weighted average Fair Isaac Corporation credit scoring model ("FICO") scores and weighted average loan-to-value ("LTV") ratio, rating or scoring, credit ratings and market spreads, as applicable. The Company assesses and recognizes OTTI in accordance with applicable accounting standards. Under these standards, if the Company determines that impairment on its debt securities exists and it has made the decision to sell the security or it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis, it recognizes the entire portion of the unrealized loss in earnings. If the Company has not made a decision to sell the security and it does not expect that it will be required to sell the security prior to the recovery of the amortized cost basis but the Company has determined that OTTI exists, it recognizes the credit-related portion of the decline in value of the security in earnings.There was no OTTI recognized in the first quarter of 2015. During the second quarter of 2015, the Company began implementing a strategy to improve the Bank's liquidity by selling non-high-quality liquid assets and reinvesting the funds into high-quality liquid assets. At June 30, 2015, 9 securities totaling a book value of $377 million in an unrealized loss position had not yet been sold. Because the Company could no longer assert that it does not have the intent to sell these securities, the Company determined that the impairment was other-than-temporary. As a result, these securities were written down to fair value, resulting in a $1.1 million OTTI charge. These securities were sold during the third quarter of 2015 for an additional loss of $1.0 million. The Company did not record any OTTI in earnings related to its investment securities in the third quarter of 2015 or in the three-month and nine-month periods endedSeptember 30, 2014.NOTE 4. INVESTMENT SECURITIES (continued)Management has concluded that the unrealized losses on its debt and equity securities for which it has not recognized OTTI (which was comprised of 277 individual securities at September 30, 2015) are temporary in nature since (1) they are not related to the underlying credit quality of the issuers, (2) the entire contractual principal and interest due on these securities is currently expected to be recoverable, (3) the Company does not intend to sell these investments at a loss and (4) it is more likely than not that the Company will not be required to sell the investments before recovery of the amortized cost basis, which may be at maturity. Accordingly, the Company has concluded that the impairment on these securities is not other-than-temporary. 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 September 30,  Nine-Month PeriodEnded September 30, 2015 2014 2015 2014 (in thousands)Proceeds from the sales of available-for-sale securities$497,182 $21,138 $2,751,790 $311,019        Gross realized gains$533 $131 $23,592 $11,547Gross realized losses(2,526) — (5,229) (67)OTTI— — (1,092) —    Net realized (losses)/gains$(1,993) $131 $17,271 $11,480The Company uses the specific identification method to determine the cost of the securities sold and the gain or loss recognized. The net loss realized for the three-month period endedSeptember 30, 2015 was primarily comprised of the sale of corporate debt securities with a book value of $63.7 million for a gain of $0.4 million, and the sale of ABS with a book value of $419.6 million for a loss of $1.4 million. The net gain for the nine-month period endedSeptember 30, 2015 was primarily comprised of the sale of state and municipal securities with a book value of $421.5 million for a gain of $12.1 million, the sale of corporate debt securities with a book value of $517.6 million for a gain of $7.0 million, and the sale of ABS with a book value of $683.9 million for a loss of $0.2 million, offset by OTTI of $1.1 million. The net gain realized for the three-month period endedSeptember 30, 2014 was primarily due to the sale of corporate debt securities with a book value of $88.4 million for a gain of $0.1 million. The net gain realized for the nine-month period endedSeptember 30, 2014 was primarily comprised of the sale of state and municipal securities with a book value of $89.0 million for a gain of $5.2 million, the sale of corporate debt securities with a book value of $434.8 million for a gain of $4.7 million, and the sale of MBS with a book value of $21.6 million for a gain of $1.3 million. Trading SecuritiesThe Company did not hold any trading securities at September 30, 2015, compared to $833.9 million held at December 31, 2014. Gains and losses on trading securities are recorded within Mortgage banking income, net, in the Company's Condensed Consolidated Statement of Operations as the Company utilized trading securities portfolio to economically hedge the MSR portfolio. The realized activity of trading gains and losses related to trading securities are as follows: Three-Month PeriodEnded September 30,  Nine-Month PeriodEnded September 30, 2015 2014 2015 2014 (in thousands)Net gains recognized during the period on trading securities$— $1,899 $6,391 $3,386Less: Net gains recognized during the period on trading securities sold during the period— 2,047 6,391 3,782Unrealized losses during the reporting period on trading securities still held at the reporting date$— $(148) $— $(396) NOTE 4. INVESTMENT SECURITIES (continued)As of September 30, 2015, the Company had no trading securities pledged as collateral. As of September 30, 2014, the Company had trading securities with an estimated fair value of $33.0 million pledged as collateral to secure the Bank's customer overnight sweep product.Other Investments Other investments at September 30, 2015 were $999.2 million, compared to $817.0 million at December 31, 2014. Other investments primarily include the Company's investment in the stock of the FHLB of Pittsburgh and the Federal Reserve Bank ("FRB") with aggregate carrying amounts of $975.6 million and $817.0 million as of September 30, 2015 and December 31, 2014, respectively. The stocks do not have readily determinable fair values because their ownership is restricted and they lack a market. The stocks can be sold back only at their par value of $100 per share and only to FHLBs or to another member institution. Accordingly, these stocks are carried at cost. During the three-month and nine-month periods endedSeptember 30, 2015, the Company purchased $208.0 million and $447.9 million of FHLB stock at par, respectively, and redeemed $105.4 million and $344.6 million of FHLB stock at par, respectively. The Company also purchased $3.7 million of FRB stock at par during the nine-month period endedSeptember 30, 2015. The Company did not purchase any FRB stock at par during the three-month period endedSeptember 30, 2015. There was no gain or loss associated with these sales. Other investments also include $23.6 million of Low Income Housing Tax Credit Investments as of September 30, 2015. The Company evaluates these investments for impairment based on the ultimate recoverability of the carrying value, rather than by recognizing temporary declines in value. |

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| - Definition
The entire disclosure for  investments in certain debt and equity securities.
+ References
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