Fair Value (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended																																																																																	6 Months Ended	12 Months Ended																																																																																																									6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Fair value measurements on a recurring basis	Dec. 31, 2012 Fair value measurements on a recurring basis	Jun. 30, 2013 Fair value measurements on a recurring basis Level 1	Dec. 31, 2012 Fair value measurements on a recurring basis Level 1	Jun. 30, 2013 Fair value measurements on a recurring basis Level 2	Dec. 31, 2012 Fair value measurements on a recurring basis Level 2	Jun. 30, 2013 Fair value measurements on a recurring basis Level 3	Dec. 31, 2012 Fair value measurements on a recurring basis Level 3	Jun. 30, 2013 Fair value measurements on a recurring basis Level 3 Real estate fund	Dec. 31, 2012 Fair value measurements on a recurring basis Level 3 Real estate fund	Jun. 30, 2013 Indemnity	Dec. 31, 2012 Indemnity	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Fixed maturities	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Fixed maturities	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis States & political subdivisions	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis States & political subdivisions	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Corporate debt securities	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Corporate debt securities	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Commercial mortgage-backed securities	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Collateralized debt obligations	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Collateralized debt obligations	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Nonredeemable preferred stock	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Nonredeemable preferred stock	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Common stock	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Common stock	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Real estate	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Real estate	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 1	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 1	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 1 Fixed maturities	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 1 Fixed maturities	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 1 States & political subdivisions	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 1 States & political subdivisions	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 1 Corporate debt securities	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 1 Corporate debt securities	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 1 Commercial mortgage-backed securities	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 1 Collateralized debt obligations	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 1 Collateralized debt obligations	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 1 Nonredeemable preferred stock	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 1 Nonredeemable preferred stock	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 1 Common stock	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 1 Common stock	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 1 Real estate	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 1 Real estate	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 2	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 2	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 2 Fixed maturities	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 2 Fixed maturities	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 2 States & political subdivisions	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 2 States & political subdivisions	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 2 Corporate debt securities	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 2 Corporate debt securities	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 2 Commercial mortgage-backed securities	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 2 Collateralized debt obligations	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 2 Collateralized debt obligations	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 2 Nonredeemable preferred stock	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 2 Nonredeemable preferred stock	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 2 Common stock	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 2 Common stock	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 2 Real estate	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 2 Real estate	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 3	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 3	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 3 Fixed maturities	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 3 Fixed maturities	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 3 States & political subdivisions	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 3 States & political subdivisions	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 3 Corporate debt securities	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 3 Corporate debt securities	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 3 Commercial mortgage-backed securities	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 3 Collateralized debt obligations	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 3 Collateralized debt obligations	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 3 Nonredeemable preferred stock	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 3 Nonredeemable preferred stock	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 3 Common stock	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 3 Common stock	Jun. 30, 2013 Indemnity Fair value measurements on a recurring basis Level 3 Real estate	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 3 Real estate	Jun. 30, 2013 Exchange	Dec. 31, 2012 Exchange	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Fixed maturities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Fixed maturities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis U.S. government & agencies	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis U.S. government & agencies	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis States & political subdivisions	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis States & political subdivisions	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Foreign government securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Foreign government securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Corporate debt securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Corporate debt securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Residential mortgage-backed securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Residential mortgage-backed securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Commercial mortgage-backed securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Commercial mortgage-backed securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Collateralized debt obligations	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Collateralized debt obligations	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Other debt securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Other debt securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Nonredeemable preferred stock	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Nonredeemable preferred stock	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Common stock	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Common stock	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Real estate	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Real estate	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 1	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 1	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 1 Fixed maturities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 1 Fixed maturities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 1 U.S. government & agencies	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 1 U.S. government & agencies	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 1 States & political subdivisions	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 1 States & political subdivisions	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 1 Foreign government securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 1 Foreign government securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 1 Corporate debt securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 1 Corporate debt securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 1 Residential mortgage-backed securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 1 Residential mortgage-backed securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 1 Commercial mortgage-backed securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 1 Commercial mortgage-backed securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 1 Collateralized debt obligations	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 1 Collateralized debt obligations	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 1 Other debt securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 1 Other debt securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 1 Nonredeemable preferred stock	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 1 Nonredeemable preferred stock	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 1 Common stock	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 1 Common stock	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 1 Real estate	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 1 Real estate	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 2	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 2	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 2 Fixed maturities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 2 Fixed maturities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 2 U.S. government & agencies	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 2 U.S. government & agencies	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 2 States & political subdivisions	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 2 States & political subdivisions	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 2 Foreign government securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 2 Foreign government securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 2 Corporate debt securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 2 Corporate debt securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 2 Residential mortgage-backed securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 2 Residential mortgage-backed securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 2 Commercial mortgage-backed securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 2 Commercial mortgage-backed securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 2 Collateralized debt obligations	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 2 Collateralized debt obligations	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 2 Other debt securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 2 Other debt securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 2 Nonredeemable preferred stock	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 2 Nonredeemable preferred stock	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 2 Common stock	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 2 Common stock	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 2 Real estate	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 2 Real estate	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 3	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 3	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 3 Fixed maturities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 3 Fixed maturities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 3 U.S. government & agencies	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 3 U.S. government & agencies	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 3 States & political subdivisions	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 3 States & political subdivisions	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 3 Foreign government securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 3 Foreign government securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 3 Corporate debt securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 3 Corporate debt securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 3 Residential mortgage-backed securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 3 Residential mortgage-backed securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 3 Commercial mortgage-backed securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 3 Commercial mortgage-backed securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 3 Collateralized debt obligations	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 3 Collateralized debt obligations	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 3 Other debt securities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 3 Other debt securities	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 3 Nonredeemable preferred stock	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 3 Nonredeemable preferred stock	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 3 Common stock	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 3 Common stock	Jun. 30, 2013 Exchange Fair value measurements on a recurring basis Level 3 Real estate	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 3 Real estate	Jun. 30, 2013 Minimum Fair value measurements on a recurring basis Level 3 Real estate	Dec. 31, 2012 Minimum Fair value measurements on a recurring basis Level 3 Real estate	Jun. 30, 2013 Maximum Fair value measurements on a recurring basis Level 3 Real estate	Dec. 31, 2012 Maximum Fair value measurements on a recurring basis Level 3 Real estate
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 9,180		$ 9,159											$ 486	$ 507	$ 486	$ 507	$ 438	$ 452	$ 179	$ 185	$ 258	$ 261	$ 3	$ 1	$ 3	$ 23	$ 29	$ 25	$ 26			$ 27	$ 30	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 2	$ 4	$ 25	$ 26			$ 457	$ 473	$ 436	$ 448	$ 179	$ 185	$ 257	$ 260	$ 3	$ 0	$ 0	$ 21	$ 25	$ 0	$ 0			$ 2	$ 4	$ 2	$ 4	$ 0	$ 0	$ 1	$ 1	$ 0	$ 1	$ 3	$ 0	$ 0	$ 0	$ 0			$ 8,694	$ 8,652	$ 8,694	$ 8,652	$ 7,725	$ 7,707	$ 184	$ 191	$ 1,274	$ 1,321	$ 23	$ 16	$ 5,882	$ 5,777	$ 221	$ 231	$ 49	$ 67	$ 23	$ 49	$ 69	$ 55	$ 647	$ 631	$ 322	$ 314			$ 553	$ 513	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 231	$ 199	$ 322	$ 314			$ 8,085	$ 8,080	$ 7,676	$ 7,648	$ 184	$ 191	$ 1,274	$ 1,321	$ 23	$ 16	$ 5,849	$ 5,734	$ 221	$ 231	$ 45	$ 67	$ 11	$ 33	$ 69	$ 55	$ 409	$ 432	$ 0	$ 0			$ 56	$ 59	$ 49	$ 59	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 33	$ 43	$ 0	$ 0	$ 4	$ 0	$ 12	$ 16	$ 0	$ 0	$ 7	$ 0	$ 0	$ 0
Trading securities																																																																																						2,724	2,417																					2,724	2,417			2,717	2,402																					2,717	2,402			0	0																					0	0			7	15																					7	15
Other investments																															20	19																0	0																0	0																20	19																											113	109																									0	0																									0	0																									113	109
Total				12,037	11,704	3,297	2,945	8,542	8,553	198	206					506	526																27	30																457	473																22	23																		11,531	11,178																									3,270	2,915																									8,085	8,080																									176	183
Other investments:
Number of real estate funds reported under fair value option												4	4
Underlying assets, liquidation period from inception of fund																																																																																																																																																																																														5 years	5 years	10 years	10 years
Contributions made to the funds	52.0	42.0																																																																																0	0.2																																																																																																									0	0.7
Distributions received from the funds	86.0	112.0																																																																																0.3	0.3																																																																																																									2.5	4.7
Unfunded commitments																																																																																		$ 1.5	$ 1.5																																																																																																									$ 4.5	$ 4.5