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Investments
 12 Months Ended
Dec. 31, 2017
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		   Fair
Value		  
(at December 31, 2017, in millions)
  Gains   Losses  

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

   $ 2,080   $ 4   $ 8   $ 2,076  

Obligations of states, municipalities and political subdivisions:

                          

Local general obligation

     13,488     444     26     13,906  

Revenue

     11,307     338     19     11,626  

State general obligation

     1,443     44     3     1,484  

Pre-refunded

     3,758     142     1     3,899  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total obligations of states, municipalities and political subdivisions

     29,996     968     49     30,915  

Debt securities issued by foreign governments

     1,505     14     10     1,509  

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

     2,334     87     11     2,410  

All other corporate bonds

     25,311     478     100     25,689  

Redeemable preferred stock

     90     5     —     95  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 61,316   $ 1,556   $ 178   $ 62,694  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​


 

 
    
   Gross Unrealized    
  
 
   Amortized
Cost		   Fair
Value		  
(at December 31, 2016, in millions)
  Gains   Losses  

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

   $ 2,031   $ 9   $ 5   $ 2,035  

Obligations of states, municipalities and political subdivisions:

                          

Local general obligation

     13,955     271     182     14,044  

Revenue

     10,910     215     147     10,978  

State general obligation

     1,717     36     22     1,731  

Pre-refunded

     4,968     190     1     5,157  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total obligations of states, municipalities and political subdivisions

     31,550     712     352     31,910  

Debt securities issued by foreign governments

     1,631     34     3     1,662  

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

     1,614     100     6     1,708  

All other corporate bonds

     22,737     508     138     23,107  

Redeemable preferred stock

     87     6     —     93  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 59,650   $ 1,369   $ 504   $ 60,515  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

        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, 2017, in millions)
  Amortized
Cost		   Fair
Value		  

Due in one year or less

   $ 4,720   $ 4,749  

Due after 1 year through 5 years

     16,948     17,283  

Due after 5 years through 10 years

     14,545     14,859  

Due after 10 years

     22,769     23,393  
​ ​ ​ ​ ​ ​ ​ ​

    58,982     60,284  

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

     2,334     2,410  
​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 61,316   $ 62,694  
​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​

        Pre-refunded bonds of $3.90 billion and $5.16 billion at December 31, 2017 and 2016, respectively, were bonds for which states or municipalities have established irrevocable trusts, almost exclusively comprised of U.S. Treasury securities and obligations of U.S. government and government agencies and authorities. These trusts were created to fund the payment of principal and interest due under the bonds.

        The Company's fixed maturity investment portfolio at December 31, 2017 and 2016 included $2.41 billion and $1.71 billion, respectively, of residential mortgage-backed securities, which include pass-through securities and collateralized mortgage obligations (CMOs). Included in the totals at December 31, 2017 and 2016 were $804 million and $563 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 $1.61 billion and $1.15 billion at December 31, 2017 and 2016, respectively. Approximately 55% and 51% of the Company's CMO holdings at December 31, 2017 and 2016, respectively, were guaranteed by or fully collateralized by securities issued by GNMA, FNMA or FHLMC. The weighted average credit rating of the $717 million and $566 million of non-guaranteed CMO holdings at December 31, 2017 and 2016, respectively, was "A1" and "Baa2," respectively. The weighted average credit rating of all of the above securities was "Aa1" and "Aa2" at December 31, 2017 and 2016, respectively.

        At December 31, 2017 and 2016, the Company held commercial mortgage-backed securities (CMBS, including FHA project loans) of $1.17 billion and $938 million, respectively, which are included in "All other corporate bonds" in the tables above. At December 31, 2017 and 2016, approximately $475 million and $290 million of these securities, respectively, or the loans backing such securities, contained guarantees by the U.S. government or a government-sponsored enterprise. The weighted average credit rating of the $693 million and $648 million of non-guaranteed securities at December 31, 2017 and 2016, respectively, was "Aaa" at both dates. The CMBS portfolio is supported by loans that are diversified across economic sectors and geographical areas. The weighted average credit rating of the CMBS portfolio was "Aaa" at both December 31, 2017 and 2016.

        At December 31, 2017 and 2016, the Company had $304 million and $286 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 $1.85 billion, $1.42 billion and $1.95 billion in 2017, 2016 and 2015, respectively. Gross gains of $42 million, $79 million and $95 million and gross losses of $38 million, $20 million and $14 million were realized on those sales in 2017, 2016 and 2015, respectively.

        At December 31, 2017 and 2016, the Company's insurance subsidiaries had $4.41 billion and $4.56 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 $35 million at both December 31, 2017 and 2016. Other investments pledged as collateral securing outstanding letters of credit had a fair value of $1 million and $3 million at December 31, 2017 and 2016, respectively. In addition, the Company utilizes Lloyd's trust deposits, whereby owned securities with a fair value of approximately $37 million and $97 million held by a wholly-owned subsidiary at December 31, 2017 and 2016, respectively, and $33 million held by TRV at December 31, 2017, were pledged into Lloyd's trust accounts to provide a portion of the capital needed to support the Company's obligations 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, 2017, in millions)
  Cost   Gains   Losses  

Public common stock

   $ 332   $ 8   $ 1   $ 339  

Non-redeemable preferred stock

     108     12     6     114  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 440   $ 20   $ 7   $ 453  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​


 

 
    
   Gross
Unrealized		    
  
 
    
   Fair
Value		  
(at December 31, 2016, in millions)
  Cost   Gains   Losses  

Public common stock

   $ 390   $ 216   $ 3   $ 603  

Non-redeemable preferred stock

     114     20     5     129  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 504   $ 236   $ 8   $ 732  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

        Proceeds from sales of equity securities classified as available for sale were $765 million, $92 million and $59 million in 2017, 2016 and 2015, respectively. Gross gains of $239 million, $17 million and $16 million and gross losses of $3 million, $3 million and $10 million were realized on those sales in 2017, 2016 and 2015, 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 $23 million, $69 million and $31 million in 2017, 2016 and 2015, respectively. Gross gains of $10 million, $7 million and $4 million were realized on those sales in 2017, 2016 and 2015, respectively, and there were no gross losses. Accumulated depreciation on real estate held for investment purposes was $364 million and $332 million at December 31, 2017 and 2016, respectively.

        Future minimum rental income on operating leases relating to the Company's real estate properties is expected to be $90 million, $85 million, $70 million, $56 million and $44 million for 2018, 2019, 2020, 2021 and 2022, respectively, and $58 million for 2023 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 71 days to maturity at December 31, 2017. The amortized cost of these securities, which totaled $4.90 billion and $4.87 billion at December 31, 2017 and 2016, 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. The Company considers an investment in a VIE in which it has a 20% or greater equity interest as a significant VIE. Neither the Company's carrying amounts nor the unfunded commitments related to these significant VIE's are material individually or in the aggregate.

Unrealized Investment Losses

        The following tables summarize, for all investments in an unrealized loss position at December 31, 2017 and 2016, 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, 2017, 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

   $ 1,150   $ 5   $ 470   $ 3   $ 1,620   $ 8  

Obligations of states, municipalities and political subdivisions

     505     2     2,959     47     3,464     49  

Debt securities issued by foreign governments

     394     6     111     4     505     10  

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

     1,021     7     250     4     1,271     11  

All other corporate bonds

     6,062     48     1,990     52     8,052     100  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total fixed maturities

     9,132     68     5,780     110     14,912     178  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Equity securities

                                      

Public common stock

     18     —     34     1     52     1  

Non-redeemable preferred stock

     3     —     56     6     59     6  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total equity securities

     21     —     90     7     111     7  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 9,153   $ 68   $ 5,870   $ 117   $ 15,023   $ 185  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

 
   Less than 12 months   12 months or longer   Total  
(at December 31, 2016, 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

   $ 1,124   $ 5   $ —   $ —   $ 1,124   $ 5  

Obligations of states, municipalities and political subdivisions

     9,781     352     12     —     9,793     352  

Debt securities issued by foreign governments

     360     3     —     —     360     3  

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

     528     5     43     1     571     6  

All other corporate bonds

     6,470     115     437     23     6,907     138  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total fixed maturities

     18,263     480     492     24     18,755     504  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Equity securities

                                      

Public common stock

     45     2     10     1     55     3  

Non-redeemable preferred stock

     2     —     59     5     61     5  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total equity securities

     47     2     69     6     116     8  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 18,310   $ 482   $ 561   $ 30   $ 18,871   $ 512  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

        At December 31, 2017, the amount of gross unrealized losses for all fixed maturity and equity investments reported at fair value for which fair value was less than 80% of amortized cost for fixed maturity investments and cost for equity investments was not significant.

Impairment Charges

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

(for the year ended December 31, in millions)
  2017   2016   2015  

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

     —     —     —  

All other corporate bonds

     4     15     13  

Redeemable preferred stock

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

Total fixed maturities

     4     15     13  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Equity securities

    
 		    
 		    
 		  

Public common stock

     9     9     37  

Non-redeemable preferred stock

     —     3     —  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total equity securities

     9     12     37  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Other investments

     1     2     2  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total

   $ 14   $ 29   $ 52  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

        The following tables present the cumulative amount of and the changes during the year in credit losses on fixed maturities held at December 31, 2017 and 2016, that were recognized in the consolidated statements of income from other-than-temporary impairments (OTTI) and for which a portion of the OTTI was recognized in other comprehensive income (loss) in the consolidated balance sheet.

Year ended December 31, 2017
(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

   $ 31   $ —   $ —   $ —   $ (2 ) $ 29  

All other corporate bonds

     54     —     1     (7 )   (2 )   46  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total fixed maturities

   $ 85   $ —   $ 1   $ (7 ) $ (4 ) $ 75  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Year ended December 31, 2016
(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

   $ 32   $ —   $ —   $ —   $ (1 ) $ 31  

All other corporate bonds

     51     13     —     (7 )   (3 )   54  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Total fixed maturities

   $ 83   $ 13   $ —   $ (7 ) $ (4 ) $ 85  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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, 2017 and 2016, other than U.S. Treasury securities and obligations of U.S. government and government agencies and authorities, 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 $1.67 billion and $1.76 billion at December 31, 2017 and 2016, 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)
  2017   2016   2015  

Gross investment income

                    

Fixed maturities

   $ 1,895   $ 1,981   $ 2,091  

Equity securities

     28     37     39  

Short-term securities

     62     29     12  

Real estate investments

     44     51     48  

Other investments

     406     242     230  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Gross investment income

     2,435     2,340     2,420  

Investment expenses

     38     38     41  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Net investment income

   $ 2,397   $ 2,302   $ 2,379  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

        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)
  2017   2016   2015  

Changes in net unrealized investment gains

                    

Fixed maturities

   $ 513   $ (915 ) $ (893 )

Equity securities

     (215 )   51     (143 )

Other investments

     4     2     2  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Change in net pre-tax unrealized gains on investment securities

     302     (862 )   (1,034 )

Related tax expense (benefit)

     78     (303 )   (357 )
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Change in net unrealized gains on investment securities

     224     (559 )   (677 )

Balance, beginning of year

     730     1,289     1,966  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Balance, end of year

   $ 954   $ 730   $ 1,289  
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

        The total impact of net unrealized gains on investment securities was $1.11 billion after-tax at December 31, 2017, which included the $954 million reported in accumulated other comprehensive income shown above, as well as $158 million included in retained earnings that was part of the impact of enactment of the Tax Cuts and Jobs Act of 2017 recorded in earnings.

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 both December 31, 2017 and 2016, the Company had $400 million notional value of open U.S. Treasury futures contracts. Net realized investment losses related to U.S. Treasury futures contracts in 2017, 2016 and 2015 were not significant.

        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 losses related to these derivatives in 2017, 2016 and 2015 were not significant.