Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	$ 15.0	$ 17.5
OTTI	6.4	8.2
Securities pledged, Amortized Cost	516.4	692.8
Equity securities, cost	69.3	67.1
Equity securities, available-for-sale	85.2	82.3
Total fixed maturities and equity securities, Amortized Cost	23,010.7	23,389.1
Gross Unrealized Capital Gains	1,398.5	1,075.5
Gross Unrealized Capital Losses	134.5	272.7
Fair Value	24,289.7	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	191.5	176.3
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	607.8	692.8
Fixed maturities, Gross Unrealized Capital Gains	108.4	95.9
Fixed maturities, Gross Unrealized Capital Losses	0.5	1.7
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	715.7	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	3.6	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	3.6	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	836.1	794.7
Fixed maturities, Gross Unrealized Capital Gains	28.4	16.3
Fixed maturities, Gross Unrealized Capital Losses	6.5	15.6
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	858.0	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	8,861.4	9,511.1
Fixed maturities, Gross Unrealized Capital Gains	706.9	533.3
Fixed maturities, Gross Unrealized Capital Losses	29.1	72.0
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	9,539.2	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	3,215.8	2,950.9
Fixed maturities, Gross Unrealized Capital Gains	126.3	89.4
Fixed maturities, Gross Unrealized Capital Losses	46.9	79.7
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	3,295.2	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	2,662.8	2,801.4
Fixed maturities, Gross Unrealized Capital Gains	170.2	120.6
Fixed maturities, Gross Unrealized Capital Losses	16.2	37.6
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	2,816.8	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	2,883.0	2,822.0
Fixed maturities, Gross Unrealized Capital Gains	141.2	113.9
Fixed maturities, Gross Unrealized Capital Losses	10.8	25.9
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	3,013.4	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	2,569.9	2,670.5
Fixed maturities, Gross Unrealized Capital Gains	131.5	129.0
Fixed maturities, Gross Unrealized Capital Losses	16.3	29.0
Embedded Derivatives	15.0	17.5
Fixed maturities, including securities pledged, Fair Value	2,700.1	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	2,087.8	2,356.7
Fixed maturities, Gross Unrealized Capital Gains	77.0	81.4
Fixed maturities, Gross Unrealized Capital Losses	14.7	27.0
Embedded Derivatives	7.7	9.3
Fixed maturities, including securities pledged, Fair Value	2,157.8	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	482.1	313.8
Fixed maturities, Gross Unrealized Capital Gains	54.5	47.6
Fixed maturities, Gross Unrealized Capital Losses	1.6	2.0
Embedded Derivatives	7.3	8.2
Fixed maturities, including securities pledged, Fair Value	542.3	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	1,272.1	1,301.2
Fixed maturities, Gross Unrealized Capital Gains	40.5	33.2
Fixed maturities, Gross Unrealized Capital Losses	10.3	16.7
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	1,302.3	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	545.3	466.4
Fixed maturities, Gross Unrealized Capital Gains	10.1	10.6
Fixed maturities, Gross Unrealized Capital Losses	0.8	1.3
Embedded Derivatives	0.0	0.0
Fixed maturities, including securities pledged, Fair Value	554.6	475.7
OTTI	2.0	2.1
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	23,457.8	24,014.8
Fixed maturities, Gross Unrealized Capital Gains	1,463.5	1,142.2
Fixed maturities, Gross Unrealized Capital Losses	137.4	279.5
Embedded Derivatives	15.0	17.5
Fixed maturities, including securities pledged, Fair Value	24,798.9	24,895.0
OTTI	6.4	8.2
Securities pledged, Amortized Cost	516.4	692.8
Securities pledged, Gross Unrealized Capital Gains	80.9	81.9
Securities pledged, Gross Unrealized Capital Losses	2.9	6.8
Securities pledged	594.4	767.9
Total fixed maturities, less securities pledged, Amortized Cost	22,941.4	23,322.0
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	1,382.6	1,060.3
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	134.5	272.7
Total fixed maturities, less securities pledged, Fair Value	24,204.5	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	69.3	67.1
Available-for-sale Equity Securities, Gross Unrealized Gain Accumulated In Investments	15.9	15.2
Available-for-sale Equity Securities, Gross Unrealized Loss Accumulated In Investments	0.0	0.0
Equity securities, available-for-sale	$ 85.2	$ 82.3