Investments (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Available-for-sale securities
Amortized cost, fixed maturities	$ 9,593
Amortized cost	10,381	$ 9,907
Gross unrealized gains	466	584
Gross unrealized losses	75	45
Estimated fair value, fixed maturities	9,933	9,571
Estimated fair value, equity securities	839	875
Estimated fair value	10,772	10,446
Indemnity
Available-for-sale securities
Amortized cost, fixed maturities	553	555
Amortized cost, equity securities	22	24
Amortized cost	575	579
Gross unrealized gains	12	14
Gross unrealized losses	4	4
Estimated fair value, fixed maturities	561	564
Estimated fair value, equity securities	22	25
Estimated fair value	583	589
Indemnity \| Fixed maturities
Available-for-sale securities
Amortized cost, fixed maturities	553	555
Gross unrealized gains, fixed maturities	11	13
Gross unrealized losses, fixed maturities	3	4
Estimated fair value, fixed maturities	561	564
Indemnity \| States & political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	215	219
Gross unrealized gains, fixed maturities	10	12
Gross unrealized losses, fixed maturities	0	0
Estimated fair value, fixed maturities	225	231
Indemnity \| Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	236	236
Gross unrealized gains, fixed maturities	1	1
Gross unrealized losses, fixed maturities	2	3
Estimated fair value, fixed maturities	235	234
Indemnity \| Residential mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	9	8
Gross unrealized gains, fixed maturities	0	0
Gross unrealized losses, fixed maturities	0	0
Estimated fair value, fixed maturities	9	8
Indemnity \| Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	50	52
Gross unrealized gains, fixed maturities	0	0
Gross unrealized losses, fixed maturities	1	1
Estimated fair value, fixed maturities	49	51
Indemnity \| Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	38	33
Gross unrealized gains, fixed maturities	0	0
Gross unrealized losses, fixed maturities	0	0
Estimated fair value, fixed maturities	38	33
Indemnity \| Other debt securities
Available-for-sale securities
Amortized cost, fixed maturities	5	7
Gross unrealized gains, fixed maturities	0	0
Gross unrealized losses, fixed maturities	0	0
Estimated fair value, fixed maturities	5	7
Indemnity \| Nonredeemable preferred stock
Available-for-sale securities
Amortized cost, equity securities	9	11
Gross unrealized gains, equity securities	1	1
Gross unrealized losses, equity securities	0	0
Estimated fair value, equity securities	10	12
Indemnity \| Common stock
Available-for-sale securities
Amortized cost, equity securities	13	13
Gross unrealized gains, equity securities	0	0
Gross unrealized losses, equity securities	1	0
Estimated fair value, equity securities	12	13
Exchange
Available-for-sale securities
Amortized cost, fixed maturities	9,040	8,540
Amortized cost, equity securities	766	788
Amortized cost	9,806	9,328
Gross unrealized gains	454	570
Gross unrealized losses	71	41
Estimated fair value, fixed maturities	9,372	9,007
Estimated fair value, equity securities	817	850
Estimated fair value	10,189	9,857
Exchange \| Fixed maturities
Available-for-sale securities
Amortized cost, fixed maturities	9,040	8,540
Gross unrealized gains, fixed maturities	399	503
Gross unrealized losses, fixed maturities	67	36
Estimated fair value, fixed maturities	9,372	9,007
Exchange \| U.S. treasury
Available-for-sale securities
Amortized cost, fixed maturities	7	6
Gross unrealized gains, fixed maturities	0	0
Gross unrealized losses, fixed maturities	0	0
Estimated fair value, fixed maturities	7	6
Exchange \| Government sponsored enterprises
Available-for-sale securities
Amortized cost, fixed maturities	3	3
Gross unrealized gains, fixed maturities	1	1
Gross unrealized losses, fixed maturities	0	0
Estimated fair value, fixed maturities	4	4
Exchange \| States & political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	1,427	1,394
Gross unrealized gains, fixed maturities	67	84
Gross unrealized losses, fixed maturities	2	1
Estimated fair value, fixed maturities	1,492	1,477
Exchange \| Foreign government securities
Available-for-sale securities
Amortized cost, fixed maturities	85	10
Gross unrealized gains, fixed maturities	0	0
Gross unrealized losses, fixed maturities	2	0
Estimated fair value, fixed maturities	83	10
Exchange \| Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	7,362	6,918
Gross unrealized gains, fixed maturities	322	405
Gross unrealized losses, fixed maturities	62	34
Estimated fair value, fixed maturities	7,622	7,289
Exchange \| Residential mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	34	109
Gross unrealized gains, fixed maturities	1	3
Gross unrealized losses, fixed maturities	1	1
Estimated fair value, fixed maturities	34	111
Exchange \| Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	28	28
Gross unrealized gains, fixed maturities	1	2
Gross unrealized losses, fixed maturities	0	0
Estimated fair value, fixed maturities	29	30
Exchange \| Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	6	6
Gross unrealized gains, fixed maturities	5	5
Gross unrealized losses, fixed maturities	0	0
Estimated fair value, fixed maturities	11	11
Exchange \| Other debt securities
Available-for-sale securities
Amortized cost, fixed maturities	88	66
Gross unrealized gains, fixed maturities	2	3
Gross unrealized losses, fixed maturities	0	0
Estimated fair value, fixed maturities	90	69
Exchange \| Nonredeemable preferred stock
Available-for-sale securities
Amortized cost, equity securities	670	650
Gross unrealized gains, equity securities	54	64
Gross unrealized losses, equity securities	3	4
Estimated fair value, equity securities	721	710
Exchange \| Common stock
Available-for-sale securities
Amortized cost, equity securities	96	138
Gross unrealized gains, equity securities	1	3
Gross unrealized losses, equity securities	1	1
Estimated fair value, equity securities	$ 96	$ 140

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