Fair Value (Assets and liabilities measured at fair value) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	$ 39,795	$ 41,487
Equity securities	773
Equity securities		695
Short term and other	1,217	1,354
Assets	41,785	43,536
Liabilities
Other liabilities	(4)	3
Liabilities	(4)	3
Corporate and other bonds
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	19,209	19,374
States, municipalities and political subdivisions
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	12,009	14,027
Total asset-backed
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	8,577	8,086
Common Stock
Assets
Equity securities	89
Equity securities		27
Preferred stock
Assets
Equity securities	684
Equity securities		668
Level 1
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	166	128
Equity securities	152
Equity securities		91
Short term and other	167	396
Assets	485	615
Liabilities
Other liabilities	0	0
Liabilities	0	0
Level 1 | Corporate and other bonds
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	166	128
Level 1 | States, municipalities and political subdivisions
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	0	0
Level 1 | Total asset-backed
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	0	0
Level 1 | Common Stock
Assets
Equity securities	85
Equity securities		23
Level 1 | Preferred stock
Assets
Equity securities	67
Equity securities		68
Level 2
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	39,261	40,925
Equity securities	604
Equity securities		584
Short term and other	1,050	958
Assets	40,915	42,467
Liabilities
Other liabilities	(4)	3
Liabilities	(4)	3
Level 2 | Corporate and other bonds
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	18,949	19,148
Level 2 | States, municipalities and political subdivisions
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	12,008	14,026
Level 2 | Total asset-backed
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	8,304	7,751
Level 2 | Common Stock
Assets
Equity securities	0
Equity securities		0
Level 2 | Preferred stock
Assets
Equity securities	604
Equity securities		584
Level 3
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	368	434
Equity securities	17
Equity securities		20
Short term and other	0	0
Assets	385	454
Liabilities
Other liabilities	0	0
Liabilities	0	0
Level 3 | Corporate and other bonds
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	94	98
Level 3 | States, municipalities and political subdivisions
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	1	1
Level 3 | Total asset-backed
Assets
Fixed maturity securities at fair value (amortized cost of $37,928 and $38,215)	273	335
Level 3 | Common Stock
Assets
Equity securities	4
Equity securities		4
Level 3 | Preferred stock
Assets
Equity securities	$ 13
Equity securities		$ 16