Fair Value (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended																																																																																	9 Months Ended	12 Months Ended																																																																																																									9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Fair value measurements on a recurring basis	Dec. 31, 2012 Fair value measurements on a recurring basis	Sep. 30, 2013 Fair value measurements on a recurring basis Level 1	Dec. 31, 2012 Fair value measurements on a recurring basis Level 1	Sep. 30, 2013 Fair value measurements on a recurring basis Level 2	Dec. 31, 2012 Fair value measurements on a recurring basis Level 2	Sep. 30, 2013 Fair value measurements on a recurring basis Level 3	Dec. 31, 2012 Fair value measurements on a recurring basis Level 3	Sep. 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	Sep. 30, 2013 Indemnity	Dec. 31, 2012 Indemnity	Sep. 30, 2013 Indemnity Fair value measurements on a recurring basis	Dec. 31, 2012 Indemnity Fair value measurements on a recurring basis	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 30, 2013 Exchange	Dec. 31, 2012 Exchange	Sep. 30, 2013 Exchange Fair value measurements on a recurring basis	Dec. 31, 2012 Exchange Fair value measurements on a recurring basis	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	$ 9,317		$ 9,159											$ 547	$ 507	$ 547	$ 507	$ 499	$ 452	$ 219	$ 185	$ 279	$ 261	$ 3	$ 1	$ 3	$ 23	$ 29	$ 25	$ 26			$ 27	$ 30	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 2	$ 4	$ 25	$ 26			$ 518	$ 473	$ 497	$ 448	$ 219	$ 185	$ 278	$ 260	$ 3	$ 0	$ 0	$ 21	$ 25	$ 0	$ 0			$ 2	$ 4	$ 2	$ 4	$ 0	$ 0	$ 1	$ 1	$ 0	$ 1	$ 3	$ 0	$ 0	$ 0	$ 0			$ 8,770	$ 8,652	$ 8,770	$ 8,652	$ 7,921	$ 7,707	$ 173	$ 191	$ 1,431	$ 1,321	$ 15	$ 16	$ 5,969	$ 5,777	$ 199	$ 231	$ 48	$ 67	$ 20	$ 49	$ 66	$ 55	$ 611	$ 631	$ 238	$ 314			$ 468	$ 513	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 230	$ 199	$ 238	$ 314			$ 8,267	$ 8,080	$ 7,886	$ 7,648	$ 173	$ 191	$ 1,431	$ 1,321	$ 15	$ 16	$ 5,943	$ 5,734	$ 199	$ 231	$ 48	$ 67	$ 11	$ 33	$ 66	$ 55	$ 381	$ 432	$ 0	$ 0			$ 35	$ 59	$ 35	$ 59	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 26	$ 43	$ 0	$ 0	$ 0	$ 0	$ 9	$ 16	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Trading securities																																																																																						2,945	2,417																					2,945	2,417			2,931	2,402																					2,931	2,402			0	0																					0	0			14	15																					14	15
Other investments																															19	19																0	0																0	0																19	19																											100	109																									0	0																									0	0																									100	109
Total				12,381	11,704	3,426	2,945	8,785	8,553	170	206					566	526																27	30																518	473																21	23																		11,815	11,178																									3,399	2,915																									8,267	8,080																									149	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	66.0	64.0																																																																																0	0.2																																																																																																									0	0.7
Distributions received from the funds	125.0	141.0																																																																																1.4	0.3																																																																																																									18.3	4.7
Unfunded commitments																																																																																		$ 1.5	$ 1.5																																																																																																									$ 4.5	$ 4.5