Fair Value (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 10,446	$ 9,557
Other investments:
Contributions made to the funds	120.0	88.0	100.0
Distributions received from the funds	197.0	198.0	201.0
Fair value measurements on a recurring basis
Consolidated fair value measurements on a recurring basis by asset class and level of input
Total	13,748	12,875
% of total assets at fair value	100.00%	100.00%
Fair value measurements on a recurring basis | Level 1
Consolidated fair value measurements on a recurring basis by asset class and level of input
Total	3,691	3,654
% of total assets at fair value	26.90%	28.40%
Fair value measurements on a recurring basis | Level 2
Consolidated fair value measurements on a recurring basis by asset class and level of input
Total	9,863	9,057
% of total assets at fair value	71.70%	70.30%
Fair value measurements on a recurring basis | Level 3
Consolidated fair value measurements on a recurring basis by asset class and level of input
Total	194	164
% of total assets at fair value	1.40%	1.30%
Fair value measurements on a recurring basis | Level 3 | Real estate
Other investments:
Number of real estate funds reported under fair value option	4	4
Fair value measurements on a recurring basis | Level 3 | Real estate | Minimum
Other investments:
Underlying assets, liquidation period from inception of fund (in years)	5 years	5 years
Fair value measurements on a recurring basis | Level 3 | Real estate | Maximum
Other investments:
Underlying assets, liquidation period from inception of fund (in years)	10 years	10 years
Indemnity
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	589	576
Indemnity | Fair value measurements on a recurring basis
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	589	576
Total	597	594
Indemnity | Fair value measurements on a recurring basis | Fixed maturities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	564	526
Indemnity | Fair value measurements on a recurring basis | States & political subdivisions
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	231	243
Indemnity | Fair value measurements on a recurring basis | Corporate debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	234	282
Indemnity | Fair value measurements on a recurring basis | Residential mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	8
Indemnity | Fair value measurements on a recurring basis | Commercial mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	51
Indemnity | Fair value measurements on a recurring basis | Collateralized debt obligations
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	33	1
Indemnity | Fair value measurements on a recurring basis | Other debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	7
Indemnity | Fair value measurements on a recurring basis | Nonredeemable preferred stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	12	25
Indemnity | Fair value measurements on a recurring basis | Common stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	13	25
Indemnity | Fair value measurements on a recurring basis | Real estate
Consolidated fair value measurements on a recurring basis by asset class and level of input
Other investments	8	18
Indemnity | Fair value measurements on a recurring basis | Level 1
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	15	27
Total	15	27
Indemnity | Fair value measurements on a recurring basis | Level 1 | Fixed maturities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 1 | States & political subdivisions
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 1 | Corporate debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 1 | Residential mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0
Indemnity | Fair value measurements on a recurring basis | Level 1 | Commercial mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0
Indemnity | Fair value measurements on a recurring basis | Level 1 | Collateralized debt obligations
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 1 | Other debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0
Indemnity | Fair value measurements on a recurring basis | Level 1 | Nonredeemable preferred stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	2	2
Indemnity | Fair value measurements on a recurring basis | Level 1 | Common stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	13	25
Indemnity | Fair value measurements on a recurring basis | Level 1 | Real estate
Consolidated fair value measurements on a recurring basis by asset class and level of input
Other investments	0	0
Indemnity | Fair value measurements on a recurring basis | Level 2
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	574	547
Total	574	547
Indemnity | Fair value measurements on a recurring basis | Level 2 | Fixed maturities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	564	524
Indemnity | Fair value measurements on a recurring basis | Level 2 | States & political subdivisions
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	231	243
Indemnity | Fair value measurements on a recurring basis | Level 2 | Corporate debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	234	281
Indemnity | Fair value measurements on a recurring basis | Level 2 | Residential mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	8
Indemnity | Fair value measurements on a recurring basis | Level 2 | Commercial mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	51
Indemnity | Fair value measurements on a recurring basis | Level 2 | Collateralized debt obligations
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	33	0
Indemnity | Fair value measurements on a recurring basis | Level 2 | Other debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	7
Indemnity | Fair value measurements on a recurring basis | Level 2 | Nonredeemable preferred stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	10	23
Indemnity | Fair value measurements on a recurring basis | Level 2 | Common stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 2 | Real estate
Consolidated fair value measurements on a recurring basis by asset class and level of input
Other investments	0	0
Indemnity | Fair value measurements on a recurring basis | Level 3
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	2
Total	8	20
Indemnity | Fair value measurements on a recurring basis | Level 3 | Fixed maturities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	2
Indemnity | Fair value measurements on a recurring basis | Level 3 | States & political subdivisions
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 3 | Corporate debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	1
Indemnity | Fair value measurements on a recurring basis | Level 3 | Residential mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0
Indemnity | Fair value measurements on a recurring basis | Level 3 | Commercial mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0
Indemnity | Fair value measurements on a recurring basis | Level 3 | Collateralized debt obligations
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	1
Indemnity | Fair value measurements on a recurring basis | Level 3 | Other debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0
Indemnity | Fair value measurements on a recurring basis | Level 3 | Nonredeemable preferred stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 3 | Common stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Indemnity | Fair value measurements on a recurring basis | Level 3 | Real estate
Consolidated fair value measurements on a recurring basis by asset class and level of input
Other investments	8	18
Other investments:
Contributions made to the funds	0	0
Distributions received from the funds	12.9	2.4
Unfunded commitments	0.6	1.5
Exchange
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	9,857	8,981
Exchange | Fair value measurements on a recurring basis
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	9,857	8,981
Trading securities	3,223	3,202
Total	13,151	12,281
Exchange | Fair value measurements on a recurring basis | Fixed maturities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	9,007	8,162
Exchange | Fair value measurements on a recurring basis | U.S. government & agencies
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		172
Exchange | Fair value measurements on a recurring basis | U.S. treasury
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	6
Exchange | Fair value measurements on a recurring basis | Government sponsored enterprises
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	4
Exchange | Fair value measurements on a recurring basis | States & political subdivisions
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	1,477	1,470
Exchange | Fair value measurements on a recurring basis | Foreign government securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	10	15
Exchange | Fair value measurements on a recurring basis | Corporate debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	7,289	6,211
Exchange | Fair value measurements on a recurring basis | Residential mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	111	156
Exchange | Fair value measurements on a recurring basis | Commercial mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	30	47
Exchange | Fair value measurements on a recurring basis | Collateralized debt obligations
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	11	16
Exchange | Fair value measurements on a recurring basis | Other debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	69	75
Exchange | Fair value measurements on a recurring basis | Nonredeemable preferred stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	710	621
Exchange | Fair value measurements on a recurring basis | Common stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	140	198
Trading securities	3,223	3,202
Exchange | Fair value measurements on a recurring basis | Real estate
Consolidated fair value measurements on a recurring basis by asset class and level of input
Other investments	71	98
Exchange | Fair value measurements on a recurring basis | Level 1
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	468	440
Trading securities	3,208	3,187
Total	3,676	3,627
Exchange | Fair value measurements on a recurring basis | Level 1 | Fixed maturities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 1 | U.S. government & agencies
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		0
Exchange | Fair value measurements on a recurring basis | Level 1 | U.S. treasury
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0
Exchange | Fair value measurements on a recurring basis | Level 1 | Government sponsored enterprises
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0
Exchange | Fair value measurements on a recurring basis | Level 1 | States & political subdivisions
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 1 | Foreign government securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 1 | Corporate debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 1 | Residential mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 1 | Commercial mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 1 | Collateralized debt obligations
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 1 | Other debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 1 | Nonredeemable preferred stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	328	242
Exchange | Fair value measurements on a recurring basis | Level 1 | Common stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	140	198
Trading securities	3,208	3,187
Exchange | Fair value measurements on a recurring basis | Level 1 | Real estate
Consolidated fair value measurements on a recurring basis by asset class and level of input
Other investments	0	0
Exchange | Fair value measurements on a recurring basis | Level 2
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	9,289	8,510
Trading securities	0	0
Total	9,289	8,510
Exchange | Fair value measurements on a recurring basis | Level 2 | Fixed maturities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	8,908	8,131
Exchange | Fair value measurements on a recurring basis | Level 2 | U.S. government & agencies
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		172
Exchange | Fair value measurements on a recurring basis | Level 2 | U.S. treasury
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	6
Exchange | Fair value measurements on a recurring basis | Level 2 | Government sponsored enterprises
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	4
Exchange | Fair value measurements on a recurring basis | Level 2 | States & political subdivisions
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	1,477	1,470
Exchange | Fair value measurements on a recurring basis | Level 2 | Foreign government securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	10	15
Exchange | Fair value measurements on a recurring basis | Level 2 | Corporate debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	7,202	6,185
Exchange | Fair value measurements on a recurring basis | Level 2 | Residential mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	111	156
Exchange | Fair value measurements on a recurring basis | Level 2 | Commercial mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	30	47
Exchange | Fair value measurements on a recurring basis | Level 2 | Collateralized debt obligations
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	11	11
Exchange | Fair value measurements on a recurring basis | Level 2 | Other debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	57	75
Exchange | Fair value measurements on a recurring basis | Level 2 | Nonredeemable preferred stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	381	379
Exchange | Fair value measurements on a recurring basis | Level 2 | Common stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Trading securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 2 | Real estate
Consolidated fair value measurements on a recurring basis by asset class and level of input
Other investments	0	0
Exchange | Fair value measurements on a recurring basis | Level 3
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	100	31
Trading securities	15	15
Total	186	144
Exchange | Fair value measurements on a recurring basis | Level 3 | Fixed maturities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	99	31
Exchange | Fair value measurements on a recurring basis | Level 3 | U.S. government & agencies
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		0
Exchange | Fair value measurements on a recurring basis | Level 3 | U.S. treasury
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0
Exchange | Fair value measurements on a recurring basis | Level 3 | Government sponsored enterprises
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0
Exchange | Fair value measurements on a recurring basis | Level 3 | States & political subdivisions
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 3 | Foreign government securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 3 | Corporate debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	87	26
Exchange | Fair value measurements on a recurring basis | Level 3 | Residential mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 3 | Commercial mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Exchange | Fair value measurements on a recurring basis | Level 3 | Collateralized debt obligations
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	5
Exchange | Fair value measurements on a recurring basis | Level 3 | Other debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	12	0
Exchange | Fair value measurements on a recurring basis | Level 3 | Nonredeemable preferred stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	1	0
Exchange | Fair value measurements on a recurring basis | Level 3 | Common stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Trading securities	15	15
Exchange | Fair value measurements on a recurring basis | Level 3 | Real estate
Consolidated fair value measurements on a recurring basis by asset class and level of input
Other investments	71	98
Other investments:
Contributions made to the funds	0	0
Distributions received from the funds	41.5	21.7
Unfunded commitments	$ 1.7	$ 4.5