Investments - Fixed Maturities and Equity Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	$ 27.5	$ 27.8
Equity securities, available-for-sale	123.6	121.9
OTTI	18.5	19.7
Securities pledged, Amortized Cost	217.1	224.4
Equity securities, cost	107.7	107.4
Total fixed maturities and equity securities, Amortized Cost	20,254.1	19,918.1
Gross Unrealized Capital Gains	1,900.6	1,663.2
Gross Unrealized Capital Losses	75.5	105.9
Fair Value	22,106.7	21,503.2
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	670.5	649.0
Fixed maturities, Gross Unrealized Capital Gains	140.1	124.1
Fixed maturities, Gross Unrealized Capital Losses	0	0
Embedded Derivatives	0	0
Fixed maturities, including securities pledged, Fair Value	810.6	773.1
OTTI	0	0
U.S. government agencies and authorities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	45.6	45.7
Fixed maturities, Gross Unrealized Capital Gains	0.6	0.9
Fixed maturities, Gross Unrealized Capital Losses	0	0
Embedded Derivatives	0	0
Fixed maturities, including securities pledged, Fair Value	46.2	46.6
OTTI	0	0
State, municipalities and political subdivisions
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	303.2	259.0
Fixed maturities, Gross Unrealized Capital Gains	22.0	18.3
Fixed maturities, Gross Unrealized Capital Losses	0.1	0.1
Embedded Derivatives	0	0
Fixed maturities, including securities pledged, Fair Value	325.1	277.2
OTTI	0	0
U.S. corporate securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	10,643.9	10,366.7
Fixed maturities, Gross Unrealized Capital Gains	1,041.8	902.4
Fixed maturities, Gross Unrealized Capital Losses	34.8	49.1
Embedded Derivatives	0	0
Fixed maturities, including securities pledged, Fair Value	11,650.9	11,220.0
OTTI	1.4	1.5
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	5,477.7	5,485.4
Fixed maturities, Gross Unrealized Capital Gains	403.3	348.5
Fixed maturities, Gross Unrealized Capital Losses	39.2	56.0
Embedded Derivatives	0	0
Fixed maturities, including securities pledged, Fair Value	5,841.8	5,777.9
OTTI	0	0
Foreign securities government
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	339.4	346.5
Fixed maturities, Gross Unrealized Capital Gains	27.5	23.8
Fixed maturities, Gross Unrealized Capital Losses	3.1	5.5
Embedded Derivatives	0	0
Fixed maturities, including securities pledged, Fair Value	363.8	364.8
OTTI	0	0
Foreign securities other
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	5,138.3	5,138.9
Fixed maturities, Gross Unrealized Capital Gains	375.8	324.7
Fixed maturities, Gross Unrealized Capital Losses	36.1	50.5
Embedded Derivatives	0	0
Fixed maturities, including securities pledged, Fair Value	5,478.0	5,413.1
OTTI	0	0
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,838.6	1,841.4
Fixed maturities, Gross Unrealized Capital Gains	189.6	180.0
Fixed maturities, Gross Unrealized Capital Losses	4.3	5.8
Embedded Derivatives	27.5	27.8
Fixed maturities, including securities pledged, Fair Value	2,051.4	2,043.4
OTTI	7.8	8.9
Residential mortgage-backed securities agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,620.0	1,613.5
Fixed maturities, Gross Unrealized Capital Gains	134.4	125.4
Fixed maturities, Gross Unrealized Capital Losses	2.3	3.6
Embedded Derivatives	15.2	15.7
Fixed maturities, including securities pledged, Fair Value	1,767.3	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	218.6	227.9
Fixed maturities, Gross Unrealized Capital Gains	55.2	54.6
Fixed maturities, Gross Unrealized Capital Losses	2.0	2.2
Embedded Derivatives	12.3	12.1
Fixed maturities, including securities pledged, Fair Value	284.1	292.4
OTTI	7.6	8.7
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,046.9	998.9
Fixed maturities, Gross Unrealized Capital Gains	99.4	79.2
Fixed maturities, Gross Unrealized Capital Losses	0.1	0.1
Embedded Derivatives	0	0
Fixed maturities, including securities pledged, Fair Value	1,146.2	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	337.1	389.0
Fixed maturities, Gross Unrealized Capital Gains	14.3	13.1
Fixed maturities, Gross Unrealized Capital Losses	1.5	1.7
Embedded Derivatives	0	0
Fixed maturities, including securities pledged, Fair Value	349.9	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	20,363.5	20,035.1
Fixed maturities, Gross Unrealized Capital Gains	1,911.1	1,666.5
Fixed maturities, Gross Unrealized Capital Losses	80.0	112.8
Embedded Derivatives	27.5	27.8
Fixed maturities, including securities pledged, Fair Value	22,222.1	21,616.6
OTTI	18.5	19.7
Securities pledged, Amortized Cost	217.1	224.4
Securities pledged, Gross Unrealized Capital Gains	26.4	17.8
Securities pledged, Gross Unrealized Capital Losses	4.5	6.9
Securities pledged, Gross Unrealized Capital Losses	239.0	235.3
Total fixed maturities, less securities pledged, Amortized Cost	20,146.4	19,810.7
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	1,884.7	1,648.7
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	75.5	105.9
Total fixed maturities, less securities pledged, Fair Value	21,983.1	21,381.3
Equity securities, available-for-sale
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	0	0
Equity securities, available-for-sale	123.6	121.9
OTTI	0	0
Equity securities, cost	107.7	107.4
Available-for-sale Equity Securities, Gross Unrealized Gain Accumulated In Investments	15.9	14.5
Available-for-sale Equity Securities, Gross Unrealized Loss Accumulated In Investments	$ 0	$ 0