Investments (Trading Account Assets Supporting Insurance Liabilities) (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost or Cost		$ 21,398	$ 21,407
Fair Value	[1]	22,126	21,840
Fixed maturities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost or Cost		19,521	19,655
Fair Value		19,954	19,850
Equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost or Cost		1,251	1,097
Fair Value		1,546	1,335
Short-term investments and cash equivalents
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost or Cost		626	655
Fair Value		626	655
Corporate securities | Fixed maturities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost or Cost		13,811	13,903
Fair Value		14,115	13,997
Commercial mortgage-backed securities | Fixed maturities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost or Cost		2,154	2,032
Fair Value		2,183	2,052
Residential mortgage-backed securities | Fixed maturities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost or Cost		999	1,142
Fair Value		1,010	1,150
Asset-backed securities | Fixed maturities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost or Cost		1,190	1,333
Fair Value		1,216	1,349
Foreign government bonds | Fixed maturities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost or Cost		1,019	915
Fair Value		1,032	926
U.S. government authorities and agencies and obligations of U.S. states | Fixed maturities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost or Cost		348	330
Fair Value		$ 398	$ 376

[1] See Note 5 for details of balances associated with variable interest entities.