Investments (Summary of fixed maturity and equity securities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Cost or Amortized Cost	$ 38,467	$ 37,444
Gross Unrealized Gain	2,744	2,717
Gross Unrealized Loss	196	392
Estimated fair value	41,015	39,769
Corporate and other bonds [Member]
Cost or Amortized Cost	17,711	17,080
Gross Unrealized Gain	1,323	1,019
Gross Unrealized Loss	76	342
Estimated fair value	18,958	17,757
Unrealized OTTI Losses (Gains)	(1)	0
States, municipalities and political subdivisions [Member]
Cost or Amortized Cost	12,060	11,729
Gross Unrealized Gain	1,213	1,453
Gross Unrealized Loss	33	8
Estimated fair value	13,240	13,174
Unrealized OTTI Losses (Gains)	(16)	(4)
Residential mortgage-backed [Member]
Cost or Amortized Cost	5,004	4,935
Gross Unrealized Gain	120	154
Gross Unrealized Loss	51	17
Estimated fair value	5,073	5,072
Unrealized OTTI Losses (Gains)	(28)	(37)
Commercial mortgage-backed [Member]
Cost or Amortized Cost	2,016	2,154
Gross Unrealized Gain	48	55
Gross Unrealized Loss	24	12
Estimated fair value	2,040	2,197
Unrealized OTTI Losses (Gains)	0	0
Other asset-backed [Member]
Cost or Amortized Cost	1,022	923
Gross Unrealized Gain	8	6
Gross Unrealized Loss	5	8
Estimated fair value	1,025	921
Unrealized OTTI Losses (Gains)	0	0
Total asset-backed [Member]
Cost or Amortized Cost	8,042	8,012
Gross Unrealized Gain	176	215
Gross Unrealized Loss	80	37
Estimated fair value	8,138	8,190
Unrealized OTTI Losses (Gains)	(28)	(37)
U.S. Treasury and obligations of government-sponsored enterprises [Member]
Cost or Amortized Cost	83	62
Gross Unrealized Gain	10	5
Gross Unrealized Loss	0	0
Estimated fair value	93	67
Unrealized OTTI Losses (Gains)	0	0
Foreign Government Debt Securities [Member]
Cost or Amortized Cost	435	334
Gross Unrealized Gain	13	13
Gross Unrealized Loss	3	1
Estimated fair value	445	346
Unrealized OTTI Losses (Gains)	0	0
Redeemable preferred stock [Member]
Cost or Amortized Cost	18	33
Gross Unrealized Gain	1	2
Gross Unrealized Loss	0	0
Estimated fair value	19	35
Unrealized OTTI Losses (Gains)	0	0
Total fixed maturity securities available-for-sale [Member]
Cost or Amortized Cost	38,349	37,250
Gross Unrealized Gain	2,736	2,707
Gross Unrealized Loss	192	388
Estimated fair value	40,893	39,569
Unrealized OTTI Losses (Gains)	(45)	(41)
Fixed Maturities [Member]
Cost or Amortized Cost, Trading Securities	12	3
Estimated Fair Value, Trading Securities	12	3
Common stock [Member]
Cost or Amortized Cost	13	46
Gross Unrealized Gain	6	3
Gross Unrealized Loss	0	1
Estimated fair value	19	48
Preferred stock [Member]
Cost or Amortized Cost	93	145
Gross Unrealized Gain	2	7
Gross Unrealized Loss	4	3
Estimated fair value	91	149
Total equity securities available-for-sale [Member]
Cost or Amortized Cost	106	191
Gross Unrealized Gain	8	10
Gross Unrealized Loss	4	4
Estimated fair value	$ 110	$ 197