Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	$ 25.1	$ 27.8
OTTI	18.0	19.7
Securities pledged, Amortized Cost	176.2	224.4
Equity securities, cost	106.6	107.4
Equity securities, available-for-sale	121.3	121.9
Total fixed maturities and equity securities, Amortized Cost	20,417.2	19,918.1
Gross Unrealized Capital Gains	1,211.0	1,663.2
Gross Unrealized Capital Losses	239.3	105.9
Fair Value	21,414.0	21,503.2
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	574.5	649.0
Fixed maturities, Gross Unrealized Capital Gains	94.1	124.1
Fixed maturities, Gross Unrealized Capital Losses	0.1	0.0
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	668.5	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	45.5	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	45.6	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	346.9	259.0
Fixed maturities, Gross Unrealized Capital Gains	10.7	18.3
Fixed maturities, Gross Unrealized Capital Losses	8.5	0.1
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	349.1	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	8,655.7	8,345.9
Fixed maturities, Gross Unrealized Capital Gains	482.3	762.9
Fixed maturities, Gross Unrealized Capital Losses	111.5	40.2
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	9,026.5	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	2,194.6	2,020.8
Fixed maturities, Gross Unrealized Capital Gains	100.8	139.5
Fixed maturities, Gross Unrealized Capital Losses	34.2	8.9
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	2,261.2	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	2,832.8	2,778.3
Fixed maturities, Gross Unrealized Capital Gains	127.0	159.1
Fixed maturities, Gross Unrealized Capital Losses	64.2	50.3
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	2,895.6	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	2,739.5	2,707.1
Fixed maturities, Gross Unrealized Capital Gains	151.3	189.4
Fixed maturities, Gross Unrealized Capital Losses	11.9	5.7
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	2,878.9	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	1,744.9	1,841.4
Fixed maturities, Gross Unrealized Capital Gains	177.2	180.0
Fixed maturities, Gross Unrealized Capital Losses	6.5	5.8
Embedded Derivatives	25.1	27.8
Fixed maturities, including securities pledged, Fair Value	1,940.7	2,043.4
OTTI	7.3	8.9
Residential mortgage-backed securities agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,545.5	1,613.5
Fixed maturities, Gross Unrealized Capital Gains	128.3	125.4
Fixed maturities, Gross Unrealized Capital Losses	4.6	3.6
Embedded Derivatives	14.0	15.7
Fixed maturities, including securities pledged, Fair Value	1,683.2	1,751.0
OTTI	0.2	0.2
Residential mortgage-backed securities non-agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	199.4	227.9
Fixed maturities, Gross Unrealized Capital Gains	48.9	54.6
Fixed maturities, Gross Unrealized Capital Losses	1.9	2.2
Embedded Derivatives	11.1	12.1
Fixed maturities, including securities pledged, Fair Value	257.5	292.4
OTTI	7.1	8.7
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,044.2	998.9
Fixed maturities, Gross Unrealized Capital Gains	55.1	79.2
Fixed maturities, Gross Unrealized Capital Losses	6.1	0.1
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	1,093.2	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	308.2	389.0
Fixed maturities, Gross Unrealized Capital Gains	12.8	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	319.6	400.4
OTTI	2.6	2.6
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	20,486.8	20,035.1
Fixed maturities, Gross Unrealized Capital Gains	1,211.4	1,666.5
Fixed maturities, Gross Unrealized Capital Losses	244.4	112.8
Embedded Derivatives	25.1	27.8
Fixed maturities, including securities pledged, Fair Value	21,478.9	21,616.6
OTTI	18.0	19.7
Securities pledged, Amortized Cost	176.2	224.4
Securities pledged, Gross Unrealized Capital Gains	15.1	17.8
Securities pledged, Gross Unrealized Capital Losses	5.1	6.9
Securities pledged, Gross Unrealized Capital Losses	186.2	235.3
Total fixed maturities, less securities pledged, Amortized Cost	20,310.6	19,810.7
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	1,196.3	1,648.7
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	239.3	105.9
Total fixed maturities, less securities pledged, Fair Value	21,292.7	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	106.6	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	$ 121.3	$ 121.9