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Fair Value (Details)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2015
USD ($)
fund
Sep. 30, 2014
USD ($)
Dec. 31, 2014
USD ($)
fund
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale securities $ 10,720.0   $ 10,446.0
Other investments:      
Contributions made to the funds 101.0 $ 82.0  
Distributions received from the funds 176.0 $ 100.0  
Fair value measurements on a recurring basis      
Fair value measurements on a recurring basis by asset class and level of input      
Total $ 13,754.0   $ 13,748.0
% 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 $ 3,388.0   $ 3,691.0
% of total assets at fair value 24.60%   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 $ 10,165.0   $ 9,863.0
% of total assets at fair value 73.90%   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 $ 201.0   $ 194.0
% of total assets at fair value 1.50%   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 | fund 4   4
Indemnity      
Fair value measurements on a recurring basis by asset class and level of input      
Available-for-sale securities $ 608.0   $ 589.0
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 608.0   589.0
Total 613.0   597.0
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 588.0   564.0
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 237.0   231.0
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 245.0   234.0
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 11.0   8.0
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 44.0   51.0
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 46.0   33.0
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 5.0   7.0
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 8.0   12.0
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 12.0   13.0
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 5.0   8.0
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 13.0   15.0
Total 13.0   15.0
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   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   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   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   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   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   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   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 1.0   2.0
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 12.0   13.0
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   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 590.0   574.0
Total 590.0   574.0
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 583.0   564.0
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 237.0   231.0
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 245.0   234.0
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 11.0   8.0
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 44.0   51.0
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 41.0   33.0
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 5.0   7.0
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 7.0   10.0
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   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   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 5.0   0.0
Total 10.0   8.0
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 5.0   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   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   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   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   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 5.0   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   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   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   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 5.0   8.0
Other investments:      
Contributions made to the funds 0.0   0.0
Distributions received from the funds 3.1   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 10,112.0   9,857.0
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 10,112.0   9,857.0
Trading securities 2,981.0   3,223.0
Total 13,141.0   13,151.0
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 9,392.0   9,007.0
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 8.0   6.0
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 4.0   4.0
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 1,476.0   1,477.0
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 86.0   10.0
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 7,657.0   7,289.0
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 30.0   111.0
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 29.0   30.0
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 11.0   11.0
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 91.0   69.0
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 623.0   710.0
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 97.0   140.0
Trading securities 2,981.0   3,223.0
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 48.0   71.0
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 407.0   468.0
Trading securities 2,968.0   3,208.0
Total 3,375.0   3,676.0
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   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   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   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   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   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   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   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   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   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   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 310.0   328.0
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 97.0   140.0
Trading securities 2,968.0   3,208.0
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   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 9,575.0   9,289.0
Trading securities 0.0   0.0
Total 9,575.0   9,289.0
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 9,268.0   8,908.0
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 8.0   6.0
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 4.0   4.0
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 1,476.0   1,477.0
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 86.0   10.0
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 7,544.0   7,202.0
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 30.0   111.0
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 29.0   30.0
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 11.0   11.0
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 80.0   57.0
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 307.0   381.0
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   0.0
Trading securities 0.0   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   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 130.0   100.0
Trading securities 13.0   15.0
Total 191.0   186.0
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 124.0   99.0
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   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   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   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   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 113.0   87.0
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   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   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   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 11.0   12.0
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 6.0   1.0
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   0.0
Trading securities 13.0   15.0
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 48.0   71.0
Other investments:      
Contributions made to the funds 0.0   0.0
Distributions received from the funds 28.4   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