Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	$ 23.7	$ 23.4
OTTI	16.1	16.9
Securities pledged, Amortized Cost	571.4	252.3
Equity securities, cost	68.0	116.7
Equity securities, available-for-sale	83.9	131.3
Total fixed maturities and equity securities, Amortized Cost	22,150.6	21,661.8
Gross Unrealized Capital Gains	1,896.0	990.8
Gross Unrealized Capital Losses	135.9	535.1
Fair Value	23,934.4	22,140.9
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	644.8	616.6
Fixed maturities, Gross Unrealized Capital Gains	173.4	105.1
Fixed maturities, Gross Unrealized Capital Losses	0.0	0.3
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	818.2	721.4
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	4.0	4.3
Fixed maturities, Gross Unrealized Capital Gains	0.2	0.0
Fixed maturities, Gross Unrealized Capital Losses	0.0	0.0
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	4.2	4.3
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	702.1	589.9
Fixed maturities, Gross Unrealized Capital Gains	64.5	13.8
Fixed maturities, Gross Unrealized Capital Losses	0.3	7.9
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	766.3	595.8
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	9,393.1	9,472.4
Fixed maturities, Gross Unrealized Capital Gains	897.9	384.9
Fixed maturities, Gross Unrealized Capital Losses	40.0	256.8
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	10,251.0	9,600.5
OTTI	1.3	1.4
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,439.0	2,336.0
Fixed maturities, Gross Unrealized Capital Gains	176.2	86.3
Fixed maturities, Gross Unrealized Capital Losses	31.5	62.4
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	2,583.7	2,359.9
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	2,754.2	2,868.7
Fixed maturities, Gross Unrealized Capital Gains	186.9	95.0
Fixed maturities, Gross Unrealized Capital Losses	54.9	151.5
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	2,886.2	2,812.2
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	2,690.0	2,678.8
Fixed maturities, Gross Unrealized Capital Gains	191.4	96.1
Fixed maturities, Gross Unrealized Capital Losses	6.6	63.5
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	2,874.8	2,711.4
OTTI	0.0	0.0
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,477.7	1,761.1
Fixed maturities, Gross Unrealized Capital Gains	172.3	151.6
Fixed maturities, Gross Unrealized Capital Losses	5.9	6.9
Embedded Derivatives	23.7	23.4
Fixed maturities, including securities pledged, Fair Value	2,667.8	1,929.2
OTTI	5.9	6.4
Residential mortgage-backed securities agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,178.1	1,579.5
Fixed maturities, Gross Unrealized Capital Gains	121.7	105.3
Fixed maturities, Gross Unrealized Capital Losses	3.1	4.8
Embedded Derivatives	13.0	12.8
Fixed maturities, including securities pledged, Fair Value	2,309.7	1,692.8
OTTI	0.0	0.0
Residential mortgage-backed securities non-agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	299.6	181.6
Fixed maturities, Gross Unrealized Capital Gains	50.6	46.3
Fixed maturities, Gross Unrealized Capital Losses	2.8	2.1
Embedded Derivatives	10.7	10.6
Fixed maturities, including securities pledged, Fair Value	358.1	236.4
OTTI	5.9	6.4
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,282.3	1,228.9
Fixed maturities, Gross Unrealized Capital Gains	118.7	49.5
Fixed maturities, Gross Unrealized Capital Losses	0.1	3.5
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	1,400.9	1,274.9
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	266.8	240.7
Fixed maturities, Gross Unrealized Capital Gains	12.3	9.9
Fixed maturities, Gross Unrealized Capital Losses	1.3	1.4
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	277.8	249.2
OTTI	2.2	2.4
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	22,654.0	21,797.4
Fixed maturities, Gross Unrealized Capital Gains	1,993.8	992.2
Fixed maturities, Gross Unrealized Capital Losses	140.6	554.2
Embedded Derivatives	23.7	23.4
Fixed maturities, including securities pledged, Fair Value	24,530.9	22,258.8
OTTI	16.1	16.9
Securities pledged, Amortized Cost	571.4	252.3
Securities pledged, Gross Unrealized Capital Gains	113.7	16.0
Securities pledged, Gross Unrealized Capital Losses	4.7	19.1
Securities pledged, Gross Unrealized Capital Losses	680.4	249.2
Total fixed maturities, less securities pledged, Amortized Cost	22,082.6	21,545.1
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	1,880.1	976.2
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	135.9	535.1
Total fixed maturities, less securities pledged, Fair Value	23,850.5	22,009.6
Equity securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	0.0	0.0
OTTI	0.0	0.0
Equity securities, cost	68.0	116.7
Available-for-sale Equity Securities, Gross Unrealized Gain Accumulated In Investments	15.9	14.6
Available-for-sale Equity Securities, Gross Unrealized Loss Accumulated In Investments	0.0	0.0
Equity securities, available-for-sale	$ 83.9	$ 131.3