Fair Value (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 10,917		$ 10,446
Other investments:
Contributions made to the funds	25.0	27.0
Distributions received from the funds	76.0	41.0
Fair value measurements on a recurring basis
Fair value measurements on a recurring basis by asset class and level of input
Total	14,071		13,748
% of total assets at fair value	100.00%		100.00%
Fair value measurements on a recurring basis | Level 1
Fair value measurements on a recurring basis by asset class and level of input
Total	3,570		3,691
% of total assets at fair value	25.40%		26.90%
Fair value measurements on a recurring basis | Level 2
Fair value measurements on a recurring basis by asset class and level of input
Total	10,338		9,863
% of total assets at fair value	73.50%		71.70%
Fair value measurements on a recurring basis | Level 3
Fair value measurements on a recurring basis by asset class and level of input
Total	163		194
% of total assets at fair value	1.10%		1.40%
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
Indemnity
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	581		589
Indemnity | Fair value measurements on a recurring basis
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	581		589
Total	588		597
Indemnity | Fair value measurements on a recurring basis | Fixed maturities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	556		564
Indemnity | Fair value measurements on a recurring basis | States & political subdivisions
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	225		231
Indemnity | Fair value measurements on a recurring basis | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	232		234
Indemnity | Fair value measurements on a recurring basis | Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	8		8
Indemnity | Fair value measurements on a recurring basis | Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	50		51
Indemnity | Fair value measurements on a recurring basis | Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	36		33
Indemnity | Fair value measurements on a recurring basis | Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	5		7
Indemnity | Fair value measurements on a recurring basis | Nonredeemable preferred stock
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	12		12
Indemnity | Fair value measurements on a recurring basis | Common stock
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	13		13
Indemnity | Fair value measurements on a recurring basis | Real estate
Fair value measurements on a recurring basis by asset class and level of input
Other investments	7		8
Indemnity | Fair value measurements on a recurring basis | Level 1
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	15		15
Total	15		15
Indemnity | Fair value measurements on a recurring basis | Level 1 | Fixed maturities
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
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
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
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 | Commercial mortgage-backed securities
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 | Collateralized debt obligations
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
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 | Nonredeemable preferred stock
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
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	13		13
Indemnity | Fair value measurements on a recurring basis | Level 1 | Real estate
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
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	566		574
Total	566		574
Indemnity | Fair value measurements on a recurring basis | Level 2 | Fixed maturities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	556		564
Indemnity | Fair value measurements on a recurring basis | Level 2 | States & political subdivisions
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	225		231
Indemnity | Fair value measurements on a recurring basis | Level 2 | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	232		234
Indemnity | Fair value measurements on a recurring basis | Level 2 | Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	8		8
Indemnity | Fair value measurements on a recurring basis | Level 2 | Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	50		51
Indemnity | Fair value measurements on a recurring basis | Level 2 | Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	36		33
Indemnity | Fair value measurements on a recurring basis | Level 2 | Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	5		7
Indemnity | Fair value measurements on a recurring basis | Level 2 | Nonredeemable preferred stock
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	10		10
Indemnity | Fair value measurements on a recurring basis | Level 2 | Common stock
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
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
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0		0
Total	7		8
Indemnity | Fair value measurements on a recurring basis | Level 3 | Fixed maturities
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 | States & political subdivisions
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
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 | Residential mortgage-backed securities
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 | Commercial mortgage-backed securities
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 | Collateralized debt obligations
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 | Other debt securities
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 | Nonredeemable preferred stock
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
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
Fair value measurements on a recurring basis by asset class and level of input
Other investments	7		8
Other investments:
Contributions made to the funds	0		0
Distributions received from the funds	0.9		12.9
Unfunded commitments	0.6		0.6
Exchange
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	10,336		9,857
Exchange | Fair value measurements on a recurring basis
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	10,336		9,857
Trading securities	3,096		3,223
Total	13,483		13,151
Exchange | Fair value measurements on a recurring basis | Fixed maturities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	9,499		9,007
Exchange | Fair value measurements on a recurring basis | U.S. treasury
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	6		6
Exchange | Fair value measurements on a recurring basis | Government sponsored enterprises
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	4		4
Exchange | Fair value measurements on a recurring basis | States & political subdivisions
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	1,506		1,477
Exchange | Fair value measurements on a recurring basis | Foreign government securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	82		10
Exchange | Fair value measurements on a recurring basis | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	7,724		7,289
Exchange | Fair value measurements on a recurring basis | Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	58		111
Exchange | Fair value measurements on a recurring basis | Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	30		30
Exchange | Fair value measurements on a recurring basis | Collateralized debt obligations
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 | Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	78		69
Exchange | Fair value measurements on a recurring basis | Nonredeemable preferred stock
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	738		710
Exchange | Fair value measurements on a recurring basis | Common stock
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	99		140
Trading securities	3,096		3,223
Exchange | Fair value measurements on a recurring basis | Real estate
Fair value measurements on a recurring basis by asset class and level of input
Other investments	51		71
Exchange | Fair value measurements on a recurring basis | Level 1
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	474		468
Trading securities	3,081		3,208
Total	3,555		3,676
Exchange | Fair value measurements on a recurring basis | Level 1 | Fixed maturities
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. treasury
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 | Government sponsored enterprises
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 | States & political subdivisions
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
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
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
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
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
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
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
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	375		328
Exchange | Fair value measurements on a recurring basis | Level 1 | Common stock
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	99		140
Trading securities	3,081		3,208
Exchange | Fair value measurements on a recurring basis | Level 1 | Real estate
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
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	9,772		9,289
Trading securities	0		0
Total	9,772		9,289
Exchange | Fair value measurements on a recurring basis | Level 2 | Fixed maturities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	9,410		8,908
Exchange | Fair value measurements on a recurring basis | Level 2 | U.S. treasury
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	6		6
Exchange | Fair value measurements on a recurring basis | Level 2 | Government sponsored enterprises
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	4		4
Exchange | Fair value measurements on a recurring basis | Level 2 | States & political subdivisions
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	1,506		1,477
Exchange | Fair value measurements on a recurring basis | Level 2 | Foreign government securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	82		10
Exchange | Fair value measurements on a recurring basis | Level 2 | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	7,644		7,202
Exchange | Fair value measurements on a recurring basis | Level 2 | Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	58		111
Exchange | Fair value measurements on a recurring basis | Level 2 | Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	30		30
Exchange | Fair value measurements on a recurring basis | Level 2 | Collateralized debt obligations
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
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	69		57
Exchange | Fair value measurements on a recurring basis | Level 2 | Nonredeemable preferred stock
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	362		381
Exchange | Fair value measurements on a recurring basis | Level 2 | Common stock
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
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
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	90		100
Trading securities	15		15
Total	156		186
Exchange | Fair value measurements on a recurring basis | Level 3 | Fixed maturities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	89		99
Exchange | Fair value measurements on a recurring basis | Level 3 | U.S. treasury
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 | Government sponsored enterprises
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 | States & political subdivisions
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
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
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	80		87
Exchange | Fair value measurements on a recurring basis | Level 3 | Residential mortgage-backed securities
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
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
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 | Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	9		12
Exchange | Fair value measurements on a recurring basis | Level 3 | Nonredeemable preferred stock
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	1		1
Exchange | Fair value measurements on a recurring basis | Level 3 | Common stock
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
Fair value measurements on a recurring basis by asset class and level of input
Other investments	51		71
Other investments:
Contributions made to the funds	0		0
Distributions received from the funds	20.7		41.5
Unfunded commitments	$ 1.7		$ 1.7
Minimum | Fair value measurements on a recurring basis | Level 3 | Real estate
Other investments:
Underlying assets, liquidation period from inception of fund (in years)	5 years		5 years
Maximum | Fair value measurements on a recurring basis | Level 3 | Real estate
Other investments:
Underlying assets, liquidation period from inception of fund (in years)	10 years		10 years