Investments (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Available-for-sale securities
Amortized cost	$ 8,889	$ 8,378
Gross unrealized gains	509	790
Gross unrealized losses	81	9
Estimated fair value, fixed maturities	8,420	8,159
Estimated fair value, equity securities	897	1,000
Estimated fair value	9,317	9,159
Indemnity
Available-for-sale securities
Amortized cost, fixed maturities	491	437
Amortized cost, equity securities	48	54
Amortized cost	539	491
Gross unrealized gains	11	17
Gross unrealized losses	3	1
Estimated fair value, fixed maturities	499	452
Estimated fair value, equity securities	48	55
Estimated fair value	547	507
Indemnity | Fixed maturities
Available-for-sale securities
Amortized cost, fixed maturities	491	437
Gross unrealized gains	10	15
Gross unrealized losses	2	0
Estimated fair value, fixed maturities	499	452
Indemnity | States & political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	212	172
Gross unrealized gains	8	13
Gross unrealized losses	1	0
Estimated fair value, fixed maturities	219	185
Indemnity | Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	278	259
Gross unrealized gains	2	2
Gross unrealized losses	1	0
Estimated fair value, fixed maturities	279	261
Indemnity | Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities		3
Gross unrealized gains		0
Gross unrealized losses		0
Estimated fair value, fixed maturities		3
Indemnity | Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	1	3
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	1	3
Indemnity | Nonredeemable preferred stock
Available-for-sale securities
Amortized cost, equity securities	22	28
Gross unrealized gains	1	2
Gross unrealized losses	0	1
Estimated fair value, equity securities	23	29
Indemnity | Common stock
Available-for-sale securities
Amortized cost, equity securities	26	26
Gross unrealized gains	0	0
Gross unrealized losses	1	0
Estimated fair value, equity securities	25	26
Exchange
Available-for-sale securities
Amortized cost, fixed maturities	7,541	7,016
Amortized cost, equity securities	809	871
Amortized cost	8,350	7,887
Gross unrealized gains	498	773
Gross unrealized losses	78	8
Estimated fair value, fixed maturities	7,921	7,707
Estimated fair value, equity securities	849	945
Estimated fair value	8,770	8,652
Exchange | Fixed maturities
Available-for-sale securities
Amortized cost, fixed maturities	7,541	7,016
Gross unrealized gains	445	696
Gross unrealized losses	65	5
Estimated fair value, fixed maturities	7,921	7,707
Exchange | U.S. government & agencies
Available-for-sale securities
Amortized cost, fixed maturities	173	190
Gross unrealized gains	2	2
Gross unrealized losses	2	1
Estimated fair value, fixed maturities	173	191
Exchange | States & political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	1,383	1,218
Gross unrealized gains	60	103
Gross unrealized losses	12	0
Estimated fair value, fixed maturities	1,431	1,321
Exchange | Foreign government securities
Available-for-sale securities
Amortized cost, fixed maturities	15	15
Gross unrealized gains	0	1
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	15	16
Exchange | Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	5,647	5,211
Gross unrealized gains	367	569
Gross unrealized losses	45	3
Estimated fair value, fixed maturities	5,969	5,777
Exchange | Residential mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	200	226
Gross unrealized gains	4	6
Gross unrealized losses	5	1
Estimated fair value, fixed maturities	199	231
Exchange | Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	46	62
Gross unrealized gains	2	5
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	48	67
Exchange | Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	13	43
Gross unrealized gains	7	6
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	20	49
Exchange | Other debt securities
Available-for-sale securities
Amortized cost, fixed maturities	64	51
Gross unrealized gains	3	4
Gross unrealized losses	1	0
Estimated fair value, fixed maturities	66	55
Exchange | Nonredeemable preferred stock
Available-for-sale securities
Amortized cost, equity securities	568	555
Gross unrealized gains	53	77
Gross unrealized losses	10	1
Estimated fair value, equity securities	611	631
Exchange | Common stock
Available-for-sale securities
Amortized cost, equity securities	241	316
Gross unrealized gains	0	0
Gross unrealized losses	3	2
Estimated fair value, equity securities	$ 238	$ 314