Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	$ 14.0	$ 17.5
OTTI	6.3	8.2
Securities pledged, Amortized Cost	944.4	692.8
Equity securities, cost	69.2	67.1
Equity securities, available-for-sale	85.0	82.3
Total fixed maturities and equity securities, Amortized Cost	22,973.9	23,389.1
Gross Unrealized Capital Gains	1,474.6	1,075.5
Gross Unrealized Capital Losses	105.6	272.7
Fair Value	24,356.9	24,209.4
Impaired available-for-sale securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Net unrealized gains on impaired available-for-sale securities	190.2	176.3
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	586.5	692.8
Fixed maturities, Gross Unrealized Capital Gains	107.2	95.9
Fixed maturities, Gross Unrealized Capital Losses	0.3	1.7
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	693.4	787.0
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	3.5	3.8
Fixed maturities, Gross Unrealized Capital Gains	0.0	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	3.5	3.8
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	843.0	794.7
Fixed maturities, Gross Unrealized Capital Gains	32.0	16.3
Fixed maturities, Gross Unrealized Capital Losses	4.9	15.6
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	870.1	795.4
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	8,734.7	9,511.1
Fixed maturities, Gross Unrealized Capital Gains	741.9	533.3
Fixed maturities, Gross Unrealized Capital Losses	22.5	72.0
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	9,454.1	9,972.4
OTTI	0.0	1.3
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	3,341.3	2,950.9
Fixed maturities, Gross Unrealized Capital Gains	141.8	89.4
Fixed maturities, Gross Unrealized Capital Losses	40.3	79.7
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	3,442.8	2,960.6
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	2,616.3	2,801.4
Fixed maturities, Gross Unrealized Capital Gains	188.9	120.6
Fixed maturities, Gross Unrealized Capital Losses	10.8	37.6
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	2,794.4	2,884.4
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	3,179.9	2,822.0
Fixed maturities, Gross Unrealized Capital Gains	161.4	113.9
Fixed maturities, Gross Unrealized Capital Losses	13.0	25.9
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	3,328.3	2,910.0
OTTI	0.0	0.0
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,610.3	2,670.5
Fixed maturities, Gross Unrealized Capital Gains	123.8	129.0
Fixed maturities, Gross Unrealized Capital Losses	14.8	29.0
Embedded Derivatives	14.0	17.5
Fixed maturities, including securities pledged, Fair Value	2,733.3	2,788.0
OTTI	4.4	4.8
Agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,009.2	2,356.7
Fixed maturities, Gross Unrealized Capital Gains	73.1	81.4
Fixed maturities, Gross Unrealized Capital Losses	13.1	27.0
Embedded Derivatives	7.1	9.3
Fixed maturities, including securities pledged, Fair Value	2,076.3	2,420.4
OTTI	0.0	0.0
Non-Agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	601.1	313.8
Fixed maturities, Gross Unrealized Capital Gains	50.7	47.6
Fixed maturities, Gross Unrealized Capital Losses	1.7	2.0
Embedded Derivatives	6.9	8.2
Fixed maturities, including securities pledged, Fair Value	657.0	367.6
OTTI	4.4	4.8
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,365.4	1,301.2
Fixed maturities, Gross Unrealized Capital Gains	39.3	33.2
Fixed maturities, Gross Unrealized Capital Losses	6.7	16.7
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	1,398.0	1,317.7
OTTI	0.0	0.0
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	568.2	466.4
Fixed maturities, Gross Unrealized Capital Gains	10.8	10.6
Fixed maturities, Gross Unrealized Capital Losses	0.6	1.3
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	578.4	475.7
OTTI	1.9	2.1
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	23,849.1	24,014.8
Fixed maturities, Gross Unrealized Capital Gains	1,547.1	1,142.2
Fixed maturities, Gross Unrealized Capital Losses	113.9	279.5
Embedded Derivatives	14.0	17.5
Fixed maturities, including securities pledged, Fair Value	25,296.3	24,895.0
OTTI	6.3	8.2
Securities pledged, Amortized Cost	944.4	692.8
Securities pledged, Gross Unrealized Capital Gains	88.3	81.9
Securities pledged, Gross Unrealized Capital Losses	8.3	6.8
Securities pledged	1,024.4	767.9
Total fixed maturities, less securities pledged, Amortized Cost	22,904.7	23,322.0
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	1,458.8	1,060.3
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	105.6	272.7
Total fixed maturities, less securities pledged, Fair Value	24,271.9	24,127.1
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	69.2	67.1
Available-for-sale Equity Securities, Gross Unrealized Gain Accumulated In Investments	15.8	15.2
Available-for-sale Equity Securities, Gross Unrealized Loss Accumulated In Investments	0.0	0.0
Equity securities, available-for-sale	$ 85.0	$ 82.3