Fair Value (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended																																																																																		3 Months Ended	12 Months Ended																																																																																																									3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Fair value measurements on a recurring basis	Dec. 31, 2012 Fair value measurements on a recurring basis	Mar. 31, 2013 Fair value measurements on a recurring basis Level 1	Dec. 31, 2012 Fair value measurements on a recurring basis Level 1	Mar. 31, 2013 Fair value measurements on a recurring basis Level 2	Dec. 31, 2012 Fair value measurements on a recurring basis Level 2	Mar. 31, 2013 Fair value measurements on a recurring basis Level 3	Dec. 31, 2012 Fair value measurements on a recurring basis Level 3	Mar. 31, 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	Mar. 31, 2012 Fair value measurements on a recurring basis Level 3 Real estate fund	Mar. 31, 2013 Indemnity	Dec. 31, 2012 Indemnity	Mar. 31, 2013 Indemnity Fair value measurements on a recurring basis	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis	Mar. 31, 2013 Indemnity Fair value measurements on a recurring basis Fixed maturities	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Fixed maturities	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 2013 Indemnity Fair value measurements on a recurring basis Common stock	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Common stock	Mar. 31, 2013 Indemnity Fair value measurements on a recurring basis Real estate	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Real estate	Mar. 31, 2013 Indemnity Fair value measurements on a recurring basis Level 1	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 1	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 2013 Indemnity Fair value measurements on a recurring basis Level 2	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 2	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 2013 Indemnity Fair value measurements on a recurring basis Level 3	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis Level 3	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 2013 Exchange	Dec. 31, 2012 Exchange	Mar. 31, 2013 Exchange Fair value measurements on a recurring basis	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis	Mar. 31, 2013 Exchange Fair value measurements on a recurring basis Fixed maturities	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Fixed maturities	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 2013 Exchange Fair value measurements on a recurring basis Common stock	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Common stock	Mar. 31, 2013 Exchange Fair value measurements on a recurring basis Real estate	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Real estate	Mar. 31, 2013 Exchange Fair value measurements on a recurring basis Level 1	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 1	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 2013 Exchange Fair value measurements on a recurring basis Level 2	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 2	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 2013 Exchange Fair value measurements on a recurring basis Level 3	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis Level 3	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	$ 9,304		$ 9,159												$ 470	$ 507	$ 470	$ 507	$ 421	$ 452	$ 176	$ 185	$ 243	$ 261	$ 3	$ 2	$ 3	$ 23	$ 29	$ 26	$ 26			$ 28	$ 30	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 2	$ 4	$ 26	$ 26			$ 439	$ 473	$ 418	$ 448	$ 176	$ 185	$ 242	$ 260	$ 3	$ 0	$ 0	$ 21	$ 25	$ 0	$ 0			$ 3	$ 4	$ 3	$ 4	$ 0	$ 0	$ 1	$ 1	$ 0	$ 2	$ 3	$ 0	$ 0	$ 0	$ 0			$ 8,834	$ 8,652	$ 8,834	$ 8,652	$ 7,891	$ 7,707	$ 185	$ 191	$ 1,299	$ 1,321	$ 29	$ 16	$ 5,974	$ 5,777	$ 234	$ 231	$ 56	$ 67	$ 46	$ 49	$ 68	$ 55	$ 631	$ 631	$ 312	$ 314			$ 548	$ 513	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 236	$ 199	$ 312	$ 314			$ 8,197	$ 8,080	$ 7,814	$ 7,648	$ 185	$ 191	$ 1,299	$ 1,321	$ 29	$ 16	$ 5,916	$ 5,734	$ 234	$ 231	$ 51	$ 67	$ 32	$ 33	$ 68	$ 55	$ 383	$ 432	$ 0	$ 0			$ 89	$ 59	$ 77	$ 59	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 58	$ 43	$ 0	$ 0	$ 5	$ 0	$ 14	$ 16	$ 0	$ 0	$ 12	$ 0	$ 0	$ 0
Trading securities																																																																																							2,662	2,417																					2,662	2,417			2,655	2,402																					2,655	2,402			0	0																					0	0			7	15																					7	15
Other investments																																20	19																0	0																0	0																20	19																											112	109																									0	0																									0	0																									112	109
Total				12,098	11,704	3,231	2,945	8,636	8,553	231	206						490	526																28	30																439	473																23	23																		11,608	11,178																									3,203	2,915																									8,197	8,080																									208	183
Other investments:
Number of real estate funds reported under fair value option												4	4	4
Underlying assets, liquidation period from inception of fund																																																																																																																																																																																															5 years	5 years	10 years	10 years
Contributions made to the funds	23.0	23.0																																																																																	0	0.2																																																																																																									0	0.7
Distributions received from the funds	53.0	81.0																																																																																	0	0.3																																																																																																									0.8	4.7
Unfunded commitments																																																																																			$ 1.5	$ 1.5																																																																																																									$ 4.5	$ 4.5