Investments (Repurchase Agreements and Securities Lending Transactions) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	$ 6,830	$ 7,882
Total securities lending transactions	5,037	3,496
Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	1,146	2,171
Total securities lending transactions	4,805	3,263
Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	5,684	5,458
Total securities lending transactions	232	233
30 to 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	253
Total securities lending transactions	0	0
Greater than 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
U.S. Treasury securities and obligations of U.S. government authorities and agencies
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	6,237	6,757
Total securities lending transactions	106	94
U.S. Treasury securities and obligations of U.S. government authorities and agencies | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	1,146	1,991
Total securities lending transactions	106	94
U.S. Treasury securities and obligations of U.S. government authorities and agencies | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	5,091	4,513
Total securities lending transactions	0	0
U.S. Treasury securities and obligations of U.S. government authorities and agencies | 30 to 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	253
Total securities lending transactions	0	0
U.S. Treasury securities and obligations of U.S. government authorities and agencies | Greater than 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Obligations of U.S. states and their political subdivisions
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	21	4
Obligations of U.S. states and their political subdivisions | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	21	4
Obligations of U.S. states and their political subdivisions | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Obligations of U.S. states and their political subdivisions | 30 to 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Obligations of U.S. states and their political subdivisions | Greater than 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Foreign government bonds
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	227	0
Foreign government bonds | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	227	0
Foreign government bonds | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Foreign government bonds | 30 to 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Foreign government bonds | Greater than 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
U.S. corporate public securities
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	11
Total securities lending transactions	3,089	1,487
U.S. corporate public securities | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	11
Total securities lending transactions	3,001	1,401
U.S. corporate public securities | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	88	86
U.S. corporate public securities | 30 to 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
U.S. corporate public securities | Greater than 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
U.S. corporate private securities
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
U.S. corporate private securities | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
U.S. corporate private securities | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
U.S. corporate private securities | 30 to 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
U.S. corporate private securities | Greater than 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Foreign Corporate Public Securities
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	910	629
Foreign Corporate Public Securities | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	849	579
Foreign Corporate Public Securities | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	61	50
Foreign Corporate Public Securities | 30 to 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Foreign Corporate Public Securities | Greater than 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Foreign Corporate Private Securities
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Foreign Corporate Private Securities | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Foreign Corporate Private Securities | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Foreign Corporate Private Securities | 30 to 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Foreign Corporate Private Securities | Greater than 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Asset-backed securities
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	241
Asset-backed securities | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	241
Asset-backed securities | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Asset-backed securities | 30 to 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Asset-backed securities | Greater than 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Commercial mortgage-backed securities
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	8
Commercial mortgage-backed securities | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	8
Commercial mortgage-backed securities | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Commercial mortgage-backed securities | 30 to 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Commercial mortgage-backed securities | Greater than 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Residential mortgage-backed securities
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	593	1,114
Total securities lending transactions	81	97
Residential mortgage-backed securities | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	169
Total securities lending transactions	0	0
Residential mortgage-backed securities | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	593	945
Total securities lending transactions	81	97
Residential mortgage-backed securities | 30 to 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Residential mortgage-backed securities | Greater than 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Equity securities
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	603	936
Equity securities | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	601	936
Equity securities | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	2	0
Equity securities | 30 to 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	0	0
Equity securities | Greater than 90 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total repurchase agreements	0	0
Total securities lending transactions	$ 0	$ 0