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Investment Securities
12 Months Ended
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]  
Investment Securities
Investment Securities
Investment securities held by us are classified as either trading, AFS, or HTM at the time of purchase and reassessed periodically, based on management’s intent.
Generally, trading assets are debt and equity securities purchased in connection with our trading activities and, as such, are expected to be sold in the near term. Our trading activities typically involve active and frequent buying and selling with the objective of generating profits on short-term movements. AFS investment securities are those securities that we intend to hold for an indefinite period of time. AFS investment securities include securities utilized as part of our asset-and-liability management activities that may be sold in response to changes in interest rates, prepayment risk, liquidity needs or other factors. HTM securities are debt securities that management has the intent and the ability to hold to maturity.
Trading assets are carried at fair value. Both realized and unrealized gains and losses on trading assets are recorded in trading services revenue in our consolidated statement of income. Debt and marketable equity securities classified as AFS are carried at fair value, and after-tax net unrealized gains and losses are recorded in AOCI. Gains or losses realized on sales of AFS investment securities are computed using the specific identification method and are recorded in gains (losses) related to investment securities, net, in our consolidated statement of income. HTM investment securities are carried at cost, adjusted for amortization of premiums and accretion of discounts.
The following table presents the amortized cost, fair value and associated unrealized gains and losses of investment securities as of the dates indicated:
 
December 31, 2017
 
December 31, 2016
 
Amortized
Cost
 
Gross
Unrealized
 
Fair
Value
 
Amortized
Cost
 
Gross
Unrealized
 
Fair
Value
(In millions)
Gains
 
Losses
 
Gains
 
Losses
 
Available-for-sale:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
U.S. Treasury and federal agencies:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Direct obligations
$
222

 
$
2

 
$
1

 
$
223

 
$
4,265

 
$
7

 
$
9

 
$
4,263

Mortgage-backed securities
10,975

 
26

 
129

 
10,872

 
13,340

 
76

 
159

 
13,257

Total U.S. Treasury and federal agencies
11,197

 
28

 
130

 
11,095

 
17,605

 
83

 
168

 
17,520

Asset-backed securities:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Student loans(1)
3,325

 
37

 
4

 
3,358

 
5,659

 
12

 
75

 
5,596

Credit cards
1,565

 
2

 
25

 
1,542

 
1,377

 

 
26

 
1,351

Sub-prime

 

 

 

 
289

 
1

 
18

 
272

Other(2)
1,440

 
7

 

 
1,447

 
895

 
10

 

 
905

Total asset-backed securities
6,330

 
46

 
29

 
6,347

 
8,220

 
23

 
119

 
8,124

Non-U.S. debt securities:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Mortgage-backed securities
6,664

 
36

 
5

 
6,695

 
6,506

 
35

 
6

 
6,535

Asset-backed securities
2,942

 
5

 

 
2,947

 
2,513

 
4

 
1

 
2,516

Government securities
10,754

 
16

 
49

 
10,721

 
5,834

 
8

 
6

 
5,836

Other(3)
6,076

 
38

 
6

 
6,108

 
5,587

 
31

 
5

 
5,613

Total non-U.S. debt securities
26,436

 
95

 
60

 
26,471

 
20,440

 
78

 
18

 
20,500

State and political subdivisions
8,929

 
245

 
23

 
9,151

 
10,233

 
201

 
112

 
10,322

Collateralized mortgage obligations
1,060

 
3

 
9

 
1,054

 
2,610

 
18

 
35

 
2,593

Other U.S. debt securities
2,563

 
12

 
15

 
2,560

 
2,481

 
18

 
30

 
2,469

U.S. equity securities
40

 
8

 
2

 
46

 
39

 
6

 
3

 
42

Non-U.S. equity securities

 

 

 

 
3

 

 

 
3

U.S. money-market mutual funds
397

 

 

 
397

 
409

 

 

 
409

Non-U.S. money-market mutual funds

 

 

 

 
16

 

 

 
16

Total
$
56,952

 
$
437

 
$
268

 
$
57,121

 
$
62,056

 
$
427

 
$
485

 
$
61,998

Held-to-maturity:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
U.S. Treasury and federal agencies:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Direct obligations
$
17,028

 
$

 
$
143

 
$
16,885

 
$
17,527

 
$
17

 
$
58

 
$
17,486

Mortgage-backed securities
16,651

 
22

 
225

 
16,448

 
10,334

 
20

 
221

 
10,133

Total U.S. Treasury and federal agencies
33,679

 
22

 
368

 
33,333

 
27,861

 
37

 
279

 
27,619

Asset-backed securities:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Student loans(1)
3,047

 
32

 
9

 
3,070

 
2,883

 
5

 
30

 
2,858

Credit cards
798

 
2

 

 
800

 
897

 
2

 

 
899

Other
1

 

 

 
1

 
35

 

 

 
35

Total asset-backed securities
3,846

 
34

 
9

 
3,871

 
3,815

 
7

 
30

 
3,792

Non-U.S. debt securities:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Mortgage-backed securities
939

 
82

 
6

 
1,015

 
1,150

 
70

 
15

 
1,205

Asset-backed securities
263

 
1

 

 
264

 
531

 

 

 
531

Government securities
474

 
2

 

 
476

 
286

 
3

 

 
289

Other
48

 

 

 
48

 
113

 
1

 

 
114

Total non-U.S. debt securities
1,724

 
85

 
6

 
1,803

 
2,080

 
74

 
15

 
2,139

Collateralized mortgage obligations
1,209

 
45

 
6

 
1,248

 
1,413

 
42

 
11

 
1,444

Total
$
40,458

 
$
186

 
$
389

 
$
40,255

 
$
35,169

 
$
160

 
$
335

 
$
34,994

 
 
 
 
(1) Primarily composed of securities guaranteed by the federal government with respect to at least 97% of defaulted principal and accrued interest on the underlying loans.
(2) As of December 31, 2017 and December 31, 2016, the fair value of other ABS was primarily composed of $1,447 million and $905 million, respectively, of CLOs.
(3) As of December 31, 2017 and December 31, 2016, the fair value of other non-U.S. debt securities was primarily composed of $3,537 million and $3,769 million, respectively, of covered bonds and $1,885 million and $988 million, as of December 31, 2017 and December 31, 2016, respectively, of corporate bonds.


Aggregate investment securities with carrying values of approximately $48 billion and $46 billion as of December 31, 2017 and December 31, 2016, respectively, were designated as pledged for public and trust deposits, short-term borrowings and for other purposes as provided by law.
In 2017, we sold $12.2 billion of AFS, primarily Agency MBS and U.S. Treasury securities in our investment portfolio, to position for the then-existing interest rate environment resulting in a pre-tax loss of $39 million.
In 2017, $496 million of Agency MBS previously classified as AFS were transferred to HTM, and in 2016, $4.9 billion, of Agency MBS and Student Loan ABS previously classified as AFS were transferred to HTM. Both transfers reflect our intent to hold these securities until their maturity. These securities were transferred at fair value, which included a net unrealized loss of $2.8 million and a net unrealized gain of $87 million as of December 31, 2017 and 2016, respectively, within accumulated other comprehensive loss which will be accreted into interest income over the remaining life of the transferred security (ranging from approximately 10 to 42 years).
The following tables present the aggregate fair values of investment securities that have been in a continuous unrealized loss position for less than 12 months, and those that have been in a continuous unrealized loss position for 12 months or longer, as of the dates indicated:
 
Less than 12 months
 
12 months or longer
 
Total
December 31, 2017
Fair
Value
 
Gross
Unrealized
Losses
 
Fair
Value
 
Gross
Unrealized
Losses
 
Fair
Value
 
Gross
Unrealized
Losses
(In millions)
 
 
 
 
 
Available-for-sale:
 
 
 
 
 
 
 
 
 
 
 
U.S. Treasury and federal agencies:
 
 
 
 
 
 
 
 
 
 
 
Direct obligations
$

 
$

 
$
67

 
$
1

 
$
67

 
$
1

Mortgage-backed securities
5,161

 
31

 
3,341

 
98

 
8,502

 
129

Total U.S. Treasury and federal agencies
5,161

 
31

 
3,408

 
99

 
8,569

 
130

Asset-backed securities:
 
 
 
 
 
 
 
 
 
 
 
Student loans

 

 
769

 
4

 
769

 
4

Credit cards
1,289

 
25

 

 

 
1,289

 
25

Total asset-backed securities
1,289


25


769


4


2,058


29

Non-U.S. debt securities:
 
 
 
 
 
 
 
 
 
 
 
Mortgage-backed securities
1,059

 
4

 
469

 
1

 
1,528

 
5

Government securities
7,629

 
48

 
68

 
1

 
7,697

 
49

Other
816

 
4

 
289

 
2

 
1,105

 
6

Total non-U.S. debt securities
9,504


56


826


4


10,330


60

State and political subdivisions
734

 
6

 
901

 
17

 
1,635

 
23

Collateralized mortgage obligations
399

 
5

 
136

 
4

 
535

 
9

Other U.S. debt securities
1,007

 
8

 
345

 
7

 
1,352

 
15

U.S. equity securities

 

 
6

 
2

 
6

 
2

Total
$
18,094

 
$
131

 
$
6,391

 
$
137

 
$
24,485

 
$
268

Held-to-maturity:
 
 
 
 
 
 
 
 
 
 
 
U.S. Treasury and federal agencies:
 
 
 
 
 
 
 
 
 
 
 
Direct obligations
$
14,439

 
$
109

 
$
2,447

 
$
34

 
$
16,886

 
$
143

Mortgage-backed securities
6,785

 
38

 
5,988

 
187

 
12,773

 
225

Total U.S. Treasury and federal agencies
21,224

 
147

 
8,435

 
221

 
29,659

 
368

Asset-backed securities:
 
 
 
 
 
 
 
 


 


Student loans
440

 
3

 
423

 
6

 
863

 
9

Total asset-backed securities
440

 
3

 
423

 
6


863


9

Non-U.S. debt securities:
 
 
 
 
 
 
 
 
 
 
 
Mortgage-backed securities

 

 
239

 
6

 
239

 
6

Total non-U.S. debt securities




239


6


239


6

Collateralized mortgage obligations

 

 
276

 
6

 
276

 
6

Total
$
21,664


$
150


$
9,373


$
239


$
31,037


$
389

 
Less than 12 months
 
12 months or longer
 
Total
December 31, 2016
Fair
Value
 
Gross
Unrealized
Losses
 
Fair
Value
 
Gross
Unrealized
Losses
 
Fair
Value
 
Gross
Unrealized
Losses
(In millions)
 
 
 
 
 
Available-for-sale:
 
 
 
 
 
 
 
 
 
 
 
U.S. Treasury and federal agencies:
 
 
 
 
 
 
 
 
 
 
 
Direct obligations
$
651

 
$
8

 
$
180

 
$
1

 
$
831

 
$
9

Mortgage-backed securities
7,072

 
131

 
1,114

 
28

 
8,186

 
159

Total U.S. Treasury and federal agencies
7,723

 
139

 
1,294

 
29

 
9,017

 
168

Asset-backed securities:
 
 
 
 
 
 
 
 
 
 
 
Student loans
54

 

 
3,745

 
75

 
3,799

 
75

Credit cards
795

 
1

 
494

 
25

 
1,289

 
26

Sub-prime
1

 

 
252

 
18

 
253

 
18

Other
75

 

 

 

 
75

 

Total asset-backed securities
925

 
1

 
4,491

 
118

 
5,416

 
119

Non-U.S. debt securities:
 
 
 
 
 
 
 
 
 
 
 
Mortgage-backed securities
442

 
1

 
893

 
5

 
1,335

 
6

Asset-backed securities
253

 

 
276

 
1

 
529

 
1

Government securities
1,314

 
6

 

 

 
1,314

 
6

Other
670

 
4

 
218

 
1

 
888

 
5

Total non-U.S. debt securities
2,679

 
11

 
1,387

 
7

 
4,066

 
18

State and political subdivisions
3,390

 
102

 
304

 
10

 
3,694

 
112

Collateralized mortgage obligations
1,259

 
31

 
162

 
4

 
1,421

 
35

Other U.S. debt securities
944

 
24

 
157

 
6

 
1,101

 
30

U.S. equity securities
8

 

 
5

 
3

 
13

 
3

Total
$
16,928

 
$
308

 
$
7,800

 
$
177

 
$
24,728

 
$
485

Held-to-maturity:
 
 
 
 
 
 
 
 
 
 
 
U.S. Treasury and federal agencies:
 
 
 
 
 
 
 
 
 
 
 
Direct obligations
$
8,891

 
$
57

 
$
86

 
$
1

 
$
8,977

 
$
58

     Mortgage-backed securities
6,838

 
221

 

 

 
6,838

 
221

Total U.S. Treasury and federal agencies
15,729

 
278

 
86

 
1

 
15,815

 
279

Asset-backed securities:
 
 
 
 
 
 
 
 
 
 
 
Student loans
705

 
9

 
1,235

 
21

 
1,940

 
30

Credit cards
33

 

 

 

 
33

 

Other
18

 

 
9

 

 
27

 

Total asset-backed securities
756

 
9

 
1,244

 
21

 
2,000

 
30

Non-U.S. debt securities:
 
 
 
 
 
 
 
 
 
 
 
Mortgage-backed securities
54

 
2

 
330

 
13

 
384

 
15

Asset-backed securities
28

 

 
35

 

 
63

 

Government securities
180

 

 

 

 
180

 

Total non-U.S. debt securities
262

 
2

 
365

 
13

 
627

 
15

Collateralized mortgage obligations
537

 
4

 
204

 
7

 
741

 
11

Total
$
17,284

 
$
293

 
$
1,899

 
$
42

 
$
19,183

 
$
335


The following table presents contractual maturities of debt investment securities by carrying amount as of December 31, 2017. The maturities of certain ABS, MBS, and CMOs are based on expected principal payments. Actual maturities may differ from these expected maturities since certain borrowers have the right to prepay obligations with or without prepayment penalties.
December 31, 2017
Under 1
Year
 
1 to 5
Years
 
6 to 10
Years
 
Over 10
Years
 
Total
(In millions)
 
 
 
 
Available-for-sale:
 
 
 
 
 
 
 
 
 
U.S. Treasury and federal agencies:
 
 
 
 
 
 
 
 
 
Direct obligations
$

 
$
12

 
$
6

 
$
205

 
$
223

Mortgage-backed securities
96

 
762

 
3,123

 
6,891

 
10,872

Total U.S. Treasury and federal agencies
96

 
774

 
3,129

 
7,096

 
11,095

Asset-backed securities:
 
 
 
 
 
 
 
 
 
Student loans
289

 
1,044

 
685

 
1,340

 
3,358

Credit cards

 
1,290

 
252

 

 
1,542

Other

 
350

 
956

 
141

 
1,447

Total asset-backed securities
289

 
2,684

 
1,893

 
1,481

 
6,347

Non-U.S. debt securities:
 
 
 
 
 
 
 
 
 
Mortgage-backed securities
551


4,502


602


1,040

 
6,695

Asset-backed securities
205


2,185


557



 
2,947

Government securities
2,195


3,201


4,448


877

 
10,721

Other
1,078


4,235


758


37

 
6,108

Total non-U.S. debt securities
4,029

 
14,123

 
6,365

 
1,954

 
26,471

State and political subdivisions
474


2,415


4,724


1,538

 
9,151

Collateralized mortgage obligations
3


145


170


736

 
1,054

Other U.S. debt securities
296


1,097


1,107


60

 
2,560

Total
$
5,187

 
$
21,238

 
$
17,388

 
$
12,865

 
$
56,678

Held-to-maturity:
 
 
 
 
 
 
 
 
 
U.S. Treasury and federal agencies:
 
 
 
 
 
 
 
 
 
Direct obligations
$
1,988


$
14,968


$
14


$
58

 
$
17,028

Mortgage-backed securities


162


1,605


14,884

 
16,651

Total U.S. Treasury and federal agencies
1,988

 
15,130

 
1,619

 
14,942

 
33,679

Asset-backed securities:










 
 
Student loans
35


245


265


2,502

 
3,047

Credit cards
178


620





 
798

Other






1

 
1

Total asset-backed securities
213

 
865

 
265

 
2,503

 
3,846

Non-U.S. debt securities:
 
 
 
 
 
 
 
 
 
Mortgage-backed securities
132


217


45


545

 
939

Asset-backed securities
26


237





 
263

Government securities
353


121





 
474

Other


48





 
48

Total non-U.S. debt securities
511

 
623

 
45

 
545

 
1,724

Collateralized mortgage obligations
8


144


343


714

 
1,209

Total
$
2,720

 
$
16,762

 
$
2,272

 
$
18,704

 
$
40,458


The following tables present gross realized gains and losses from sales of AFS investment securities, and the components of net impairment losses included in net gains and losses related to investment securities for the periods indicated.
 
 
Years Ended December 31,
(In millions)
 
2017
 
2016
 
2015
Gross realized gains from sales of AFS investment securities
 
$
74

 
$
15

 
$
57

Gross realized losses from sales of AFS investment securities
 
(113
)
 
(5
)
 
(62
)
Net impairment losses:
 
 
 
 
 
 
Gross losses from OTTI
 

 
(2
)
 
(1
)
Losses reclassified (from) to other comprehensive income
 

 
(1
)
 

Net impairment losses(1)
 

 
(3
)
 
(1
)
Gains (losses) related to investment securities, net
 
$
(39
)
 
$
7

 
$
(6
)
(1) Net impairment losses, recognized in our consolidated statement of income, were composed of the following:
 
 
 
 
 
 
Impairment associated with expected credit losses
 
$

 
$
(1
)
 
$

Impairment associated with adverse changes in timing of expected future cash flows
 

 
(2
)
 
(1
)
Net impairment losses
 
$

 
$
(3
)
 
$
(1
)
The following table presents a roll-forward with respect to net impairment losses that have been recognized in income for the periods indicated.
 
 
Years Ended December 31,
(In millions)
 
2017
 
2016
 
2015
Balance, beginning of period
 
$
66

 
$
92

 
$
115

Additions:
 
 
 
 
 
 
Losses for which OTTI was previously recognized
 

 
2

 
1

Deductions:
 
 
 
 
 
 
Previously recognized losses related to securities sold or matured
 
(2
)
 
(28
)
 
(24
)
Balance, end of period
 
$
64

 
$
66

 
$
92


Interest income related to debt securities is recognized in our consolidated statement of income using the effective interest method, or on a basis approximating a level rate of return over the contractual or estimated life of the security. The level rate of return considers any non-refundable fees or costs, as well as purchase premiums or discounts, resulting in amortization or accretion, accordingly.
For certain debt securities acquired which are considered to be beneficial interests in securitized financial assets, the excess of our estimate of undiscounted future cash flows from these securities over their initial recorded investment is accreted into interest income on a level-yield basis over the securities’ estimated remaining terms. Subsequent decreases in these securities’ expected future cash flows are either recognized prospectively through an adjustment of the yields on the securities over their remaining terms, or are evaluated for OTTI. Increases in expected future cash flows are recognized prospectively over the securities’ estimated remaining terms through the recalculation of their yields.
Impairment
We conduct periodic reviews of individual securities to assess whether OTTI exists. Impairment exists when the current fair value of an individual security is below its amortized cost basis. For AFS and HTM debt securities, impairment is recorded in our consolidated statement of income when management intends to sell (or may be required to sell) the securities before they recover in value, or when management expects the present value of cash flows expected to be collected from the securities to be less than the amortized cost of the impaired security (a credit loss).
Our review of impaired securities generally includes:
the identification and evaluation of securities that have indications of potential OTTI, such as issuer-specific concerns, including deteriorating financial condition or bankruptcy;
the analysis of expected future cash flows of securities, based on quantitative and qualitative factors;
the analysis of the collectability of those future cash flows, including information about past events, current conditions, and reasonable and supportable forecasts;
the analysis of the underlying collateral for MBS and ABS;
the analysis of individual impaired securities, including consideration of the length of time the security has been in an unrealized loss position, the anticipated recovery period, and the magnitude of the overall price decline;
evaluation of factors or triggers that could cause individual securities to be deemed OTTI and those that would not support OTTI; and
documentation of the results of these analyses.
Factors considered in determining whether impairment is other than temporary include:
certain macroeconomic drivers;
certain industry-specific drivers;
the length of time the security has been impaired;
the severity of the impairment;
the cause of the impairment and the financial condition and near-term prospects of the issuer;
activity in the market with respect to the issuer's securities, which may indicate adverse credit conditions; and
our intention not to sell, and the likelihood that we will not be required to sell, the security for a period of time sufficient to allow for its recovery in value.
Substantially all of our investment securities portfolio is composed of debt securities. A critical component of our assessment of OTTI of these debt securities is the identification of credit-impaired securities for which management does not expect to receive cash flows sufficient to recover the entire amortized cost basis of the security.
Debt securities that are not deemed to be credit-impaired are subject to additional management analysis to assess whether management intends to sell, or, more likely than not, would be required to sell, the security before the expected recovery of its amortized cost basis.
The following provides a description of our process for the identification and assessment of OTTI, as well as information about OTTI recorded in 2017, 2016 and 2015 and changes in period-end unrealized losses, for major security types as of December 31, 2017.
U.S. Agency Securities
Our portfolio of U.S. agency direct obligations and MBS receives the implicit or explicit backing of the U.S. government in conjunction with specified financial support of the U.S. Treasury. We recorded no OTTI on these securities in 2017, 2016 or 2015. The overall increase in the unrealized losses on these securities as of December 31, 2017 was primarily attributable to interest rate increases in 2017.
Asset-Backed Securities - Student Loans
Asset-backed securities collateralized by student loans are primarily composed of securities collateralized by FFELP loans. FFELP loans benefit from a federal government guarantee of at least 97% of defaulted principal and accrued interest, with additional credit support provided to our securities in the form of over-collateralization, subordination and excess spread, which collectively total in excess of 100%. Accordingly, the vast majority of FFELP loan-backed securities are protected from traditional consumer credit risk.
We recorded no OTTI on these securities in 2017, 2016 or 2015.The improvement in the mark to market for our FFELP loan-backed securities portfolio as of December 31, 2017 was primarily attributable to the tightening FFELP spreads.
Our assessment of OTTI of these securities considers, among many other factors, the strength of the U.S. government guarantee, the performance of the underlying collateral, and the remaining average term of the FFELP loan-backed securities portfolio, which was approximately 4.6 years as of December 31, 2017.
In general, the rating agencies have largely completed their downgrade review of FFELP loan-backed securities due to potential extension of student loan repayments beyond the securities’ legal final maturity dates. At this time, we do not expect a significant number of additional downgrades related to potential legal final maturity breaches. Based on the limited price impact on the overall FFELP loan-backed securities portfolio and recent remedial actions by issuers, including amending loan-backed securities’ maturity dates and exercising cleanup calls, the credit quality of the FFELP loan-backed securities portfolio remains stable and we, as a bondholder, remain protected from principal loss as a result of the aforementioned federal government guarantee and over-collateralization. Downside risks remain should remedial actions fail to address the extension risks.
Our total exposure to private student loan-backed securities was less than $70 million as of December 31, 2017. Our assessment of OTTI of private student loan-backed securities considers, among other factors, the impact of high unemployment rates on the collateral performance of private student loans. We recorded no OTTI on these securities in 2017, 2016 or 2015.
Non-U.S. Mortgage- and Asset-Backed Securities
Non-U.S. mortgage- and asset-backed securities are primarily composed of U.K., Australian and Dutch securities collateralized by residential mortgages and German and U.K. securities collateralized by automobile loans and leases. Our assessment of impairment with respect to these securities considers the location of the underlying collateral, collateral enhancement and structural features, expected credit losses under base-case and stressed conditions and the macroeconomic outlook for the country in which the collateral is located, including housing prices and unemployment. Where appropriate, any potential loss after consideration of the above-referenced factors is further evaluated to determine whether any OTTI exists.
We recorded OTTI of less than $1 million, $2 million and $1 million in 2017, 2016 and 2015, respectively, on non-U.S. residential MBS, which resulted from adverse changes in the timing of expected future cash flows from the securities.
Our assessment of OTTI of these securities takes into account government intervention in the corresponding mortgage markets and assumes a baseline macroeconomic environment for this region, factoring in slower economic growth and continued government austerity measures. In addition, we perform stress testing and sensitivity analysis in order to understand the impact of more severe assumptions on potential OTTI.
State and Political Subdivisions and Other U.S. Debt Securities
Our municipal securities portfolio primarily includes securities issued by U.S. states and their municipalities. A portion of this portfolio is held in connection with our tax-exempt investment program, more fully described in Note 14. Our portfolio of other U.S. debt securities is primarily composed of securities issued by U.S. corporations. 
Our assessment of OTTI of these portfolios considers, among other factors, adverse conditions specifically related to the industry, geographic area or financial condition of the issuer; the structure of the security, including collateral, if any, and payment schedule; rating agency changes to the security's credit rating; the volatility of the fair value changes; and our intent and ability to hold the security until its recovery in value.  If the impairment of the security is credit-related, we estimate the future cash flows from the security, tailored to the security and considering the above-described factors, and any resulting impairment deemed to be other-than-temporary is recorded in our consolidated statement of income.  
We recorded no OTTI on these securities in 2017, 2016 or 2015. The decline in the unrealized losses on these securities as of December 31, 2017 was primarily attributable to the narrowing of spreads and U.S. Treasury rates in 2017.
U.S. Non-Agency Residential Mortgage-Backed Securities
We assess OTTI of our portfolio of U.S. non-agency residential mortgage-backed securities using cash flow models, tailored for each security, that estimate the future cash flows from the underlying mortgages, using the security-specific collateral and transaction structure. Estimates of future cash flows are subject to management judgment. The future cash flows and performance of our portfolio of U.S. non-agency residential mortgage-backed securities are a function of a number of factors, including, but not limited to, the condition of the U.S. economy, the condition of the U.S. residential mortgage markets, and the level of loan defaults, prepayments and loss severities. Management's estimates of future losses for each security also consider the underwriting and historical performance of each specific security, the underlying collateral type, vintage, borrower profile, third-party guarantees, current levels of subordination, geography and other factors.
We recorded no OTTI on these securities in 2017, 2016 or 2015.
U.S. Non-Agency Commercial Mortgage-Backed Securities
With respect to our portfolio of U.S. non-agency commercial mortgage-backed securities, OTTI is assessed by considering a number of factors, including, but not limited to, the condition of the U.S. economy and the condition of the U.S. commercial real estate market, as well as capitalization rates. Management estimates of future losses for each security also consider the underlying collateral type, property location, vintage, debt-service coverage ratios, expected property income, servicer advances and estimated property values, as well as current levels of subordination. We recorded no OTTI on these securities in 2017 and 2015. In 2016 we recorded $1 million of OTTI on these securities, all associated with expected credit losses.
The estimates, assumptions and other risk factors utilized in our assessment of impairment as described above are used by management to identify securities which are subject to further analysis of potential credit losses. Additional analyses are performed using more stressful assumptions to further evaluate the sensitivity of losses relative to the above-described factors. However, since the assumptions are based on the unique characteristics of each security, management uses a range of estimates for prepayment speeds, default, and loss severity forecasts that reflect the collateral profile of the securities within each asset class. In addition, in measuring expected credit losses, the individual characteristics of each security are examined to determine whether any additional factors would increase or mitigate the expected loss. Once losses are determined, the timing of the loss will also affect the ultimate OTTI, since the loss is ultimately subject to a discount commensurate with the purchase yield of the security.
After a review of the investment portfolio, taking into consideration current economic conditions, adverse situations that might affect our ability to fully collect principal and interest, the timing of future payments, the credit quality and performance of the collateral underlying MBS and ABS and other relevant factors, management considers the aggregate decline in fair value of the investment securities portfolio and the resulting gross pre-tax unrealized losses of $657 million related to 1,283 securities as of December 31, 2017 to be temporary, and not the result of any material changes in the credit characteristics of the securities.