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Investments
 12 Months Ended
Dec. 31, 2015
Investments disclosure  
Investments disclsoure [Text Block]

3. INVESTMENTS

Fixed Maturities

        The amortized cost and fair value of investments in fixed maturities classified as available for sale were as follows:

 
    
   Gross Unrealized    
  
 
   Amortized
Cost		    
  
(at December 31, 2015, in millions)
  Gains   Losses   Fair Value  

U.S. Treasury securities and obligations of U.S. government and government agencies and authorities

   $ 2,202   $ 8   $ 16   $ 2,194  

Obligations of states, municipalities and political subdivisions:

                          

Local general obligation

     12,744     577     3     13,318  

Revenue

     9,492     472     4     9,960  

State general obligation

     1,978     97     2     2,073  

Pre-refunded

     5,813     247     —     6,060  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total obligations of states, municipalities and political subdivisions

     30,027     1,393     9     31,411  

Debt securities issued by foreign governments

     1,829     45     1     1,873  

Mortgage-backed securities, collateralized mortgage obligations and pass-through securities

     1,863     124     6     1,981  

All other corporate bonds

     22,854     523     288     23,089  

Redeemable preferred stock

     103     7     —     110  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 58,878   $ 2,100   $ 320   $ 60,658  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​


 

 
    
   Gross Unrealized    
  
 
   Amortized
Cost		    
  
(at December 31, 2014, in millions)
  Gains   Losses   Fair Value  

U.S. Treasury securities and obligations of U.S. government and government agencies and authorities

   $ 2,022   $ 36   $ 5   $ 2,053  

Obligations of states, municipalities and political subdivisions:

                          

Local general obligation

     12,366     644     5     13,005  

Revenue

     9,833     575     4     10,404  

State general obligation

     2,467     137     1     2,603  

Pre-refunded

     7,229     332     —     7,561  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total obligations of states, municipalities and political subdivisions

     31,895     1,688     10     33,573  

Debt securities issued by foreign governments

     2,320     48     —     2,368  

Mortgage-backed securities, collateralized mortgage obligations and pass-through securities

     2,052     165     4     2,213  

All other corporate bonds

     22,390     844     99     23,135  

Redeemable preferred stock

     122     10     —     132  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 60,801   $ 2,791   $ 118   $ 63,474  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

        The amortized cost and fair value of fixed maturities by contractual maturity follow. Actual maturities will differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.

(at December 31, 2015, in millions)
  Amortized
Cost		   Fair
Value		  

Due in one year or less

   $ 6,240   $ 6,324  

Due after 1 year through 5 years

     16,741     17,296  

Due after 5 years through 10 years

     16,008     16,260  

Due after 10 years

     18,026     18,797  
​ ​ ​ ​ ​ ​ ​ ​

 

     57,015     58,677  

Mortgage-backed securities, collateralized mortgage obligations and pass-through securities

     1,863     1,981  
​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 58,878   $ 60,658  
​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​

        Pre-refunded bonds of $6.06 billion and $7.56 billion at December 31, 2015 and 2014, respectively, were bonds for which states or municipalities have established irrevocable trusts, almost exclusively comprised of U.S. Treasury securities, which were created to satisfy their responsibility for payments of principal and interest.

        The Company's fixed maturity investment portfolio at December 31, 2015 and 2014 included $1.98 billion and $2.21 billion, respectively, of residential mortgage-backed securities, which include pass-through securities and collateralized mortgage obligations (CMOs). Included in the totals at December 31, 2015 and 2014 were $676 million and $872 million, respectively, of GNMA, FNMA, FHLMC (excluding FHA project loans) and Canadian government guaranteed residential mortgage-backed pass-through securities classified as available for sale. Also included in those totals were residential CMOs classified as available for sale with a fair value of $1.30 billion and $1.34 billion at December 31, 2015 and 2014, respectively. Approximately 48% and 46% of the Company's CMO holdings at December 31, 2015 and 2014, respectively, were guaranteed by or fully collateralized by securities issued by GNMA, FNMA or FHLMC. The average credit rating of the $683 million and $725 million of non-guaranteed CMO holdings at December 31, 2015 and 2014, respectively, was "Baa2" and "Ba1," respectively. The average credit rating of all of the above securities was "Aa3" at both December 31, 2015 and 2014.

        At December 31, 2015 and 2014, the Company held commercial mortgage-backed securities (CMBS, including FHA project loans) of $865 million and $715 million, respectively, which are included in "All other corporate bonds" in the tables above. At December 31, 2015 and 2014, approximately $303 million and $202 million of these securities, respectively, or the loans backing such securities, contained guarantees by the U.S. government or a government-sponsored enterprise. The average credit rating of the $562 million and $513 million of non-guaranteed securities at December 31, 2015 and 2014, respectively, was "Aaa" at both dates. The CMBS portfolio is supported by loans that are diversified across economic sectors and geographical areas. The average credit rating of the CMBS portfolio was "Aaa" at both December 31, 2015 and 2014.

        At December 31, 2015 and 2014, the Company had $269 million and $296 million, respectively, of securities on loan as part of a tri-party lending agreement.

        Proceeds from sales of fixed maturities classified as available for sale were $1.95 billion, $1.05 billion and $1.64 billion in 2015, 2014 and 2013, respectively. Gross gains of $95 million, $44 million and $66 million and gross losses of $14 million, $12 million and $25 million were realized on those sales in 2015, 2014 and 2013, respectively.

        At December 31, 2015 and 2014, the Company's insurance subsidiaries had $4.66 billion and $4.78 billion, respectively, of securities on deposit at financial institutions in certain states pursuant to the respective states' insurance regulatory requirements. Funds deposited with third parties to be used as collateral to secure various liabilities on behalf of insureds, cedants and other creditors had a fair value of $28 million and $39 million at December 31, 2015 and 2014, respectively. Other investments pledged as collateral securing outstanding letters of credit had a fair value of $21 million and $22 million at December 31, 2015 and 2014, respectively. In addition, the Company utilized a Lloyd's trust deposit at December 31, 2015 and 2014, whereby owned securities with a fair value of approximately $140 million and $151 million, respectively, held by an insurance subsidiary were pledged into a Lloyd's trust account to support capital requirements for the Company's operations at Lloyd's.

Equity Securities

        The cost and fair value of investments in equity securities were as follows:

 
    
   Gross
Unrealized		    
  
 
    
   Fair
Value		  
(at December 31, 2015, in millions)
  Cost   Gains   Losses  

Public common stock

   $ 386   $ 164   $ 7   $ 543  

Non-redeemable preferred stock

     142     26     6     162  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 528   $ 190   $ 13   $ 705  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​


 

 
    
   Gross
Unrealized		    
  
 
    
   Fair
Value		  
(at December 31, 2014, in millions)
  Cost   Gains   Losses  

Public common stock

   $ 400   $ 295   $ 4   $ 691  

Non-redeemable preferred stock

     179     31     2     208  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 579   $ 326   $ 6   $ 899  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

        Proceeds from sales of equity securities classified as available for sale were $59 million, $158 million and $86 million in 2015, 2014 and 2013, respectively. Gross gains of $16 million, $27 million and $16 million and gross losses of $10 million, $3 million and $1 million were realized on those sales in 2015, 2014 and 2013, respectively.

Real Estate

        The Company's real estate investments include warehouses, office buildings and other commercial land and properties that are directly owned. The Company negotiates commercial leases with individual tenants through unrelated, licensed real estate brokers. Negotiated terms and conditions include, among others, rental rates, length of lease period and improvements to the premises to be provided by the landlord.

        Proceeds from the sale of real estate investments were $31 million, $15 million and $18 million in 2015, 2014 and 2013, respectively. Gross gains of $4 million, $6 million and $7 million were realized on those sales in 2015, 2014 and 2013, respectively, and there were no gross losses. The Company had no real estate held for sale at December 31, 2015 and 2014. Accumulated depreciation on real estate held for investment purposes was $320 million and $290 million at December 31, 2015 and 2014, respectively.

        Future minimum rental income on operating leases relating to the Company's real estate properties is expected to be $92 million, $74 million, $61 million, $49 million and $36 million for 2016, 2017, 2018, 2019 and 2020, respectively, and $59 million for 2021 and thereafter.

Short-term Securities

        The Company's short-term securities consist of Aaa-rated registered money market funds, U.S. Treasury securities, high-quality commercial paper (primarily A1/P1) and high-quality corporate securities purchased within a year to their maturity with a combined average of 67 days to maturity at December 31, 2015. The amortized cost of these securities, which totaled $4.67 billion and $4.36 billion at December 31, 2015 and 2014, respectively, approximated their fair value.

Variable Interest Entities

        Entities which do not have sufficient equity at risk to allow the entity to finance its activities without additional financial support or in which the equity investors, as a group, do not have the characteristic of a controlling financial interest are referred to as variable interest entities (VIE). A VIE is consolidated by the variable interest holder that is determined to have the controlling financial interest (primary beneficiary) as a result of having both the power to direct the activities of a VIE that most significantly impact the VIE's economic performance and the obligation to absorb losses or right to receive benefits from the VIE that could potentially be significant to the VIE. The Company determines whether it is the primary beneficiary of an entity subject to consolidation based on a qualitative assessment of the VIE's capital structure, contractual terms, nature of the VIE's operations and purpose and the Company's relative exposure to the related risks of the VIE on the date it becomes initially involved in the VIE. The Company reassesses its VIE determination with respect to an entity on an ongoing basis.

        The Company is a passive investor in limited partner equity interests issued by third party VIEs. These include certain of the Company's investments in private equity limited partnerships, hedge funds and real estate partnerships where the Company is not related to the general partner. These investments are generally accounted for under the equity method and reported in the Company's consolidated balance sheet as other investments unless the Company is deemed the primary beneficiary. These equity interests generally cannot be redeemed. Distributions from these investments are received by the Company as a result of liquidation of the underlying investments of the funds and/or as income distribution. The Company's maximum exposure to loss with respect to these investments is limited to the investment carrying amounts reported in the Company's consolidated balance sheet and any unfunded commitment. Neither the carrying amounts nor the unfunded commitments related to these VIEs are material.

Unrealized Investment Losses

        The following tables summarize, for all investments in an unrealized loss position at December 31, 2015 and 2014, the aggregate fair value and gross unrealized loss by length of time those securities have been continuously in an unrealized loss position. The fair value amounts reported in the tables are estimates that are prepared using the process described in note 4. The Company also relies upon estimates of several factors in its review and evaluation of individual investments, using the process described in note 1, in determining whether such investments are other-than-temporarily impaired.

 
   Less than 12 months   12 months or longer   Total  
(at December 31, 2015, in millions)
  Fair
Value		   Gross
Unrealized
Losses		   Fair
Value		   Gross
Unrealized
Losses		   Fair
Value		   Gross
Unrealized
Losses		  

Fixed maturities

                                      

U.S. Treasury securities and obligations of U.S. government and government agencies and authorities

   $ 1,820   $ 15   $ 28   $ 1   $ 1,848   $ 16  

Obligations of states, municipalities and political subdivisions

     928     7     142     2     1,070     9  

Debt securities issued by foreign governments

     172     1     —     —     172     1  

Mortgage-backed securities, collateralized mortgage obligations and pass-through securities

     473     4     57     2     530     6  

All other corporate bonds

     7,725     197     710     91     8,435     288  

Redeemable preferred stock

     8     —     —     —     8     —  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total fixed maturities

     11,126     224     937     96     12,063     320  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Equity securities

                                      

Public common stock

     48     6     33     1     81     7  

Non-redeemable preferred stock

     47     3     38     3     85     6  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total equity securities

     95     9     71     4     166     13  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 11,221   $ 233   $ 1,008   $ 100   $ 12,229   $ 333  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​


 

 
   Less than 12 months   12 months or longer   Total  
(at December 31, 2014, in millions)
  Fair
Value		   Gross
Unrealized
Losses		   Fair
Value		   Gross
Unrealized
Losses		   Fair
Value		   Gross
Unrealized
Losses		  

Fixed maturities

                                      

U.S. Treasury securities and obligations of U.S. government and government agencies and authorities

   $ 180   $ 2   $ 125   $ 3   $ 305   $ 5  

Obligations of states, municipalities and political subdivisions

     173     1     797     9     970     10  

Debt securities issued by foreign governments

     50     —     24     —     74     —  

Mortgage-backed securities, collateralized mortgage obligations and pass-through securities

     68     —     192     4     260     4  

All other corporate bonds

     2,148     38     2,355     61     4,503     99  

Redeemable preferred stock

     —     —     —     —     —     —  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total fixed maturities

     2,619     41     3,493     77     6,112     118  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Equity securities

                                      

Public common stock

     81     4     1     —     82     4  

Non-redeemable preferred stock

     44     1     42     1     86     2  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total equity securities

     125     5     43     1     168     6  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 2,744   $ 46   $ 3,536   $ 78   $ 6,280   $ 124  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

        The following table summarizes, for all fixed maturities and equity securities reported at fair value for which fair value is less than 80% of amortized cost at December 31, 2015, the gross unrealized investment loss by length of time those securities have continuously been in an unrealized loss position of greater than 20% of amortized cost:

 
   Period For Which Fair Value Is Less Than 80% of Amortized Cost  
(in millions)
  3 Months
or Less		   Greater Than
3 Months,
6 Months
or Less		   Greater Than
6 Months,
12 Months
or Less		   Greater Than
12 Months		   Total  

Fixed maturities

                                

Mortgage-backed securities

   $ —   $ —   $ —   $ —   $ —  

Other

     51     17     6     7     81  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total fixed maturities

     51     17     6     7     81  

Equity securities

     3     1     —     —     4  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 54   $ 18   $ 6   $ 7   $ 85  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

        These unrealized losses at December 31, 2015 represented less than 1% of the combined fixed maturity and equity security portfolios on a pretax basis and less than 1% of shareholders' equity on an after-tax basis.

Impairment Charges

        Impairment charges included in net realized investment gains in the consolidated statement of income were as follows:

(for the year ended December 31, in millions)
  2015   2014   2013  

Fixed maturities

                    

U.S. Treasury securities and obligations of U.S. government and government agencies and authorities

   $ —   $ —   $ —  

Obligations of states, municipalities and political subdivisions

     —     —     —  

Debt securities issued by foreign governments

     —     —     —  

Mortgage-backed securities, collateralized mortgage obligations and pass-through securities

     —     1     2  

All other corporate bonds

     13     15     3  

Redeemable preferred stock

     —     —     —  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total fixed maturities

     13     16     5  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Equity securities

    
 		    
 		    
 		  

Public common stock

     37     9     5  

Non-redeemable preferred stock

     —     —     —  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total equity securities

     37     9     5  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Other investments

     2     1     5  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 52   $ 26   $ 15  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

        The following tables present the cumulative amount of and the changes during the reporting period in the credit losses of other-than-temporary impairments (OTTI) on fixed maturities recognized in the consolidated statement of income for which a portion of the OTTI was recognized in other comprehensive income:

Year ended December 31, 2015
(in millions)
  Cumulative
OTTI Credit
Losses
Recognized for
Securities Held,
Beginning of
Period		   Additions for
OTTI Securities
Where No
Credit Losses
Were
Previously
Recognized		   Additions for
OTTI Securities
Where Credit
Losses Have
Been
Previously
Recognized		   Reductions
Due to
Sales/Defaults
of Credit-
Impaired
Securities		   Adjustments to
Book Value
of Credit-
Impaired
Securities due
to Changes in
Cash Flows		   Cumulative OTTI
Credit Losses
Recognized for
Securities Still
Held, End of
Period		  

Fixed maturities

                                      

Mortgage-backed securities, collateralized mortgage obligations and pass-through securities

   $ 40   $ —   $ —   $ (6 ) $ (2 ) $ 32  

All other corporate bonds

     59     2     —     (4 )   (6 )   51  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total fixed maturities

   $ 99   $ 2   $ —   $ (10 ) $ (8 ) $ 83  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​


 

Year ended December 31, 2014
(in millions)
  Cumulative
OTTI Credit
Losses
Recognized for
Securities Held,
Beginning of
Period		   Additions for
OTTI Securities
Where No
Credit Losses
Were
Previously
Recognized		   Additions for
OTTI Securities
Where Credit
Losses Have
Been
Previously
Recognized		   Reductions
Due to
Sales/Defaults
of Credit-
Impaired
Securities		   Adjustments to
Book Value
of Credit-
Impaired
Securities due
to Changes in
Cash Flows		   Cumulative OTTI
Credit Losses
Recognized for
Securities Still
Held, End of
Period		  

Fixed maturities

                                      

Mortgage-backed securities, collateralized mortgage obligations and pass-through securities

   $ 53   $ —   $ 1   $ (5 ) $ (9 ) $ 40  

All other corporate bonds

     65     —     3     (6 )   (3 )   59  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total fixed maturities

   $ 118   $ —   $ 4   $ (11 ) $ (12 ) $ 99  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Concentrations and Credit Quality

        Concentrations of credit risk arise from exposure to counterparties that are engaged in similar activities and have similar economic characteristics that could cause their ability to meet contractual obligations to be similarly affected by changes in economic or other conditions. The Company seeks to mitigate credit risk by actively monitoring the creditworthiness of counterparties, obtaining collateral as deemed appropriate and applying controls that include credit approvals, limits of credit exposure and other monitoring procedures.

        At December 31, 2015 and 2014, other than U.S. Treasury securities, obligations of U.S. government and government agencies and authorities and obligations of the Canadian government, the Company was not exposed to any concentration of credit risk of a single issuer greater than 5% of the Company's shareholders' equity.

        Included in fixed maturities are below investment grade securities totaling $1.71 billion and $1.91 billion at December 31, 2015 and 2014, respectively. The Company defines its below investment grade securities as those securities rated below investment grade by external rating agencies, or the equivalent by the Company when a public rating does not exist. Such securities include below investment grade bonds that are publicly traded and certain other privately issued bonds that are classified as below investment grade loans.

Net Investment Income

(for the year ended December 31, in millions)
  2015   2014   2013  

Gross investment income

                    

Fixed maturities

   $ 2,091   $ 2,244   $ 2,310  

Equity securities

     39     40     31  

Short-term securities

     12     9     11  

Real estate investments

     48     44     37  

Other investments

     230     489     364  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Gross investment income

     2,420     2,826     2,753  

Investment expenses

     41     39     37  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Net investment income

   $ 2,379   $ 2,787   $ 2,716  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

        Changes in net unrealized gains on investment securities that are included as a separate component of other comprehensive income (loss) were as follows:

(at and for the year ended December 31, in millions)
  2015   2014   2013  

Changes in net unrealized investment gains

                    

Fixed maturities

   $ (893 ) $ 913   $ (2,804 )

Equity securities

     (143 )   63     74  

Other investments

     2     2     (1 )
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Change in net pretax unrealized gains on investment securities

     (1,034 )   978     (2,731 )

Related tax expense (benefit)

     (357 )   334     (950 )
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Change in net unrealized gains on investment securities

     (677 )   644     (1,781 )

Balance, beginning of year

     1,966     1,322     3,103  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Balance, end of year

   $ 1,289   $ 1,966   $ 1,322  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Derivative Financial Instruments

        From time to time, the Company enters into U.S. Treasury note futures contracts to modify the effective duration of specific assets within the investment portfolio. U.S. Treasury futures contracts require a daily mark-to-market and settlement with the broker. At December 31, 2015 and 2014, the Company had $400 million and $350 million notional value of open U.S. Treasury futures contracts, respectively. Net realized investment gains in 2015, 2014 and 2013 included net losses of $5 million, net losses of $1 million and net gains of $115 million, respectively, related to U.S. Treasury futures contracts.

        The Company purchases investments that have embedded derivatives, primarily convertible debt securities. These embedded derivatives are carried at fair value with changes in value reflected in net realized investment gains. Derivatives embedded in convertible debt securities are reported on a combined basis with their host instrument and are classified as fixed maturities. The Company also sells a small amount of U.S. equity index put option contracts that are settled for cash upon their expiration or when they are rolled over. Net realized investment gains (losses) related to these derivatives in 2015, 2014 and 2013 were not significant.