Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	$ 25.8	$ 27.8
OTTI	17.4	19.7
Securities pledged, Amortized Cost	261.8	224.4
Equity securities, cost	117.4	107.4
Equity securities, available-for-sale	132.1	121.9
Total fixed maturities and equity securities, Amortized Cost	21,159.2	19,918.1
Gross Unrealized Capital Gains	1,219.2	1,663.2
Gross Unrealized Capital Losses	350.7	105.9
Fair Value	22,053.5	21,503.2
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	792.0	649.0
Fixed maturities, Gross Unrealized Capital Gains	120.0	124.1
Fixed maturities, Gross Unrealized Capital Losses	0.0	0.0
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	912.0	773.1
OTTI	0.0	0.0
U.S. government agencies and authorities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	4.3	45.7
Fixed maturities, Gross Unrealized Capital Gains	0.1	0.9
Fixed maturities, Gross Unrealized Capital Losses	0.0	0.0
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	4.4	46.6
OTTI	0.0	0.0
State, municipalities and political subdivisions
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	494.6	259.0
Fixed maturities, Gross Unrealized Capital Gains	14.9	18.3
Fixed maturities, Gross Unrealized Capital Losses	6.1	0.1
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	503.4	277.2
OTTI	0.0	0.0
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	9,048.5	8,345.9
Fixed maturities, Gross Unrealized Capital Gains	476.4	762.9
Fixed maturities, Gross Unrealized Capital Losses	168.1	40.2
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	9,356.8	9,068.6
OTTI	1.4	1.5
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,278.8	2,020.8
Fixed maturities, Gross Unrealized Capital Gains	106.6	139.5
Fixed maturities, Gross Unrealized Capital Losses	41.0	8.9
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	2,344.4	2,151.4
OTTI	0.0	0.0
Foreign corporate public securities and foreign governments
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,798.6	2,778.3
Fixed maturities, Gross Unrealized Capital Gains	114.0	159.1
Fixed maturities, Gross Unrealized Capital Losses	120.6	50.3
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	2,792.0	2,887.1
OTTI	0.0	0.0
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,752.9	2,707.1
Fixed maturities, Gross Unrealized Capital Gains	137.1	189.4
Fixed maturities, Gross Unrealized Capital Losses	26.1	5.7
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	2,863.9	2,890.8
OTTI	0.0	0.0
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,738.7	1,841.4
Fixed maturities, Gross Unrealized Capital Gains	173.5	180.0
Fixed maturities, Gross Unrealized Capital Losses	4.3	5.8
Embedded Derivatives	25.8	27.8
Fixed maturities, including securities pledged, Fair Value	1,933.7	2,043.4
OTTI	6.9	8.9
Residential mortgage-backed securities agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,543.7	1,613.5
Fixed maturities, Gross Unrealized Capital Gains	123.2	125.4
Fixed maturities, Gross Unrealized Capital Losses	2.3	3.6
Embedded Derivatives	14.2	15.7
Fixed maturities, including securities pledged, Fair Value	1,678.8	1,751.0
OTTI	0.0	0.2
Residential mortgage-backed securities non-agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	195.0	227.9
Fixed maturities, Gross Unrealized Capital Gains	50.3	54.6
Fixed maturities, Gross Unrealized Capital Losses	2.0	2.2
Embedded Derivatives	11.6	12.1
Fixed maturities, including securities pledged, Fair Value	254.9	292.4
OTTI	6.9	8.7
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,115.7	998.9
Fixed maturities, Gross Unrealized Capital Gains	68.8	79.2
Fixed maturities, Gross Unrealized Capital Losses	1.1	0.1
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	1,183.4	1,078.0
OTTI	6.7	6.7
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	279.5	389.0
Fixed maturities, Gross Unrealized Capital Gains	13.5	13.1
Fixed maturities, Gross Unrealized Capital Losses	1.4	1.7
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	291.6	400.4
OTTI	2.4	2.6
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	21,303.6	20,035.1
Fixed maturities, Gross Unrealized Capital Gains	1,224.9	1,666.5
Fixed maturities, Gross Unrealized Capital Losses	368.7	112.8
Embedded Derivatives	25.8	27.8
Fixed maturities, including securities pledged, Fair Value	22,185.6	21,616.6
OTTI	17.4	19.7
Securities pledged, Amortized Cost	261.8	224.4
Securities pledged, Gross Unrealized Capital Gains	20.4	17.8
Securities pledged, Gross Unrealized Capital Losses	18.0	6.9
Securities pledged, Gross Unrealized Capital Losses	264.2	235.3
Total fixed maturities, less securities pledged, Amortized Cost	21,041.8	19,810.7
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	1,204.5	1,648.7
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	350.7	105.9
Total fixed maturities, less securities pledged, Fair Value	21,921.4	21,381.3
Equity securities, available-for-sale
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	0.0	0.0
OTTI	0.0	0.0
Equity securities, cost	117.4	107.4
Available-for-sale Equity Securities, Gross Unrealized Gain Accumulated In Investments	14.7	14.5
Available-for-sale Equity Securities, Gross Unrealized Loss Accumulated In Investments	0.0	0.0
Equity securities, available-for-sale	$ 132.1	$ 121.9