Investments (Repurchase Agreements and Securities Lending Transactions) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	$ 9,540	$ 8,400
Total cash collateral for loaned securities	4,307	4,354
Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	8,765	912
Total cash collateral for loaned securities	4,146	4,319
Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	775	7,488
Total cash collateral for loaned securities	161	35
30 days or greater
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	0
Total cash collateral for loaned securities	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 securities sold under agreements to repurchase	9,169	8,260
Total cash collateral for loaned securities	303	122
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 securities sold under agreements to repurchase	8,394	911
Total cash collateral for loaned securities	142	87
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 securities sold under agreements to repurchase	775	7,349
Total cash collateral for loaned securities	161	35
Obligations of U.S. states and their political subdivisions
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total cash collateral for loaned securities	139	103
Obligations of U.S. states and their political subdivisions | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total cash collateral for loaned securities	139	103
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 cash collateral for loaned securities	0	0
Foreign government bonds
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total cash collateral for loaned securities	368	335
Foreign government bonds | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total cash collateral for loaned securities	368	335
Foreign government bonds | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total cash collateral for loaned securities	0	0
U.S. corporate public securities
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	20	1
Total cash collateral for loaned securities	2,749	2,961
U.S. corporate public securities | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	20	1
Total cash collateral for loaned securities	2,749	2,961
U.S. corporate public securities | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	0
Total cash collateral for loaned securities	0	0
Foreign corporate public securities
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	0
Total cash collateral for loaned securities	612	655
Foreign corporate public securities | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	0
Total cash collateral for loaned securities	612	655
Foreign corporate public securities | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	0
Total cash collateral for loaned securities	0	0
Residential mortgage-backed securities
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	351	139
Total cash collateral for loaned securities	0	0
Residential mortgage-backed securities | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	351	0
Total cash collateral for loaned securities	0	0
Residential mortgage-backed securities | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	139
Total cash collateral for loaned securities	0	0
Equity securities
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	0
Total cash collateral for loaned securities	136	178
Equity securities | Overnight & Continuous
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	0
Total cash collateral for loaned securities	136	178
Equity securities | Up to 30 Days
Transfer Of Certain Financial Assets Accounted For As Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	0
Total cash collateral for loaned securities	$ 0	$ 0