Investments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Available-for-sale securities
Amortized cost	$ 9,241	$ 9,147
Gross unrealized gains	563	492
Gross unrealized losses	36	82
Estimated fair value, fixed maturities	8,881	8,688
Estimated fair value, equity securities	887	869
Estimated fair value	9,768	9,557
Indemnity
Available-for-sale securities
Amortized cost, fixed maturities	490	518
Amortized cost, equity securities	35	50
Amortized cost	525	568
Gross unrealized gains	13	11
Gross unrealized losses	1	3
Estimated fair value, fixed maturities	502	526
Estimated fair value, equity securities	35	50
Estimated fair value	537	576
Indemnity | Fixed maturities
Available-for-sale securities
Amortized cost, fixed maturities	490	518
Gross unrealized gains	12	9
Gross unrealized losses	0	1
Estimated fair value, fixed maturities	502	526
Indemnity | States & political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	231	237
Gross unrealized gains	10	7
Gross unrealized losses	0	1
Estimated fair value, fixed maturities	241	243
Indemnity | Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	259	280
Gross unrealized gains	2	2
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	261	282
Indemnity | Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	0	1
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	0	1
Indemnity | Nonredeemable preferred stock
Available-for-sale securities
Amortized cost, equity securities	14	24
Gross unrealized gains	1	2
Gross unrealized losses	0	1
Estimated fair value, equity securities	15	25
Indemnity | Common stock
Available-for-sale securities
Amortized cost, equity securities	21	26
Gross unrealized gains	0	0
Gross unrealized losses	1	1
Estimated fair value, equity securities	20	25
Exchange
Available-for-sale securities
Amortized cost, fixed maturities	7,923	7,801
Amortized cost, equity securities	793	778
Amortized cost	8,716	8,579
Gross unrealized gains	550	481
Gross unrealized losses	35	79
Estimated fair value, fixed maturities	8,379	8,162
Estimated fair value, equity securities	852	819
Estimated fair value	9,231	8,981
Exchange | Fixed maturities
Available-for-sale securities
Amortized cost, fixed maturities	7,923	7,801
Gross unrealized gains	485	426
Gross unrealized losses	29	65
Estimated fair value, fixed maturities	8,379	8,162
Exchange | U.S. government & agencies
Available-for-sale securities
Amortized cost, fixed maturities		171
Gross unrealized gains		1
Gross unrealized losses		0
Estimated fair value, fixed maturities		172
Exchange | U.S. treasury
Available-for-sale securities
Amortized cost, fixed maturities	66
Gross unrealized gains	0
Gross unrealized losses	0
Estimated fair value, fixed maturities	66
Exchange | Government sponsored enterprises
Available-for-sale securities
Amortized cost, fixed maturities	37
Gross unrealized gains	1
Gross unrealized losses	0
Estimated fair value, fixed maturities	38
Exchange | States & political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	1,422	1,430
Gross unrealized gains	70	55
Gross unrealized losses	7	15
Estimated fair value, fixed maturities	1,485	1,470
Exchange | Foreign government securities
Available-for-sale securities
Amortized cost, fixed maturities	15	15
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	15	15
Exchange | Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	6,113	5,902
Gross unrealized gains	400	354
Gross unrealized losses	19	45
Estimated fair value, fixed maturities	6,494	6,211
Exchange | Residential mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	151	157
Gross unrealized gains	3	3
Gross unrealized losses	3	4
Estimated fair value, fixed maturities	151	156
Exchange | Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	42	45
Gross unrealized gains	2	2
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	44	47
Exchange | Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	7	8
Gross unrealized gains	7	8
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	14	16
Exchange | Other debt securities
Available-for-sale securities
Amortized cost, fixed maturities	70	73
Gross unrealized gains	2	3
Gross unrealized losses	0	1
Estimated fair value, fixed maturities	72	75
Exchange | Nonredeemable preferred stock
Available-for-sale securities
Amortized cost, equity securities	596	577
Gross unrealized gains	64	55
Gross unrealized losses	4	11
Estimated fair value, equity securities	656	621
Exchange | Common stock
Available-for-sale securities
Amortized cost, equity securities	197	201
Gross unrealized gains	1	0
Gross unrealized losses	2	3
Estimated fair value, equity securities	$ 196	$ 198