Fair Value of Financial Instruments - Reconciliation of Level 3 Financial Assets (Details) (Level 3, Fair Value, Measurements, Nonrecurring, USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
U.S. government and agency
Level 3 Financial Assets:
Balance, beginning of period	$ 31.4	$ 26.7	$ 25.9	$ 31.7
Purchases	18.2	2.0	23.7	2.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	1.3	1.2	1.4	3.3
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Realized and unrealized gains (losses) included in income	0	0	0	0
Unrealized gains (losses) included in OCI	(1.2)	0.2	(1.1)	(2.7)
Total	47.1	27.7	47.1	27.7
State and political subdivision
Level 3 Financial Assets:
Balance, beginning of period	98.2	78.7	116.1	48.7
Purchases	13.8	0	13.8	14.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0.4	0.2	0.6	0.4
Transfers into Level 3	0	2.4	0	15.3
Transfers out of Level 3	0	0	0	0
Realized and unrealized gains (losses) included in income	0	0	0	0
Unrealized gains (losses) included in OCI	(7.3)	9.2	(25.0)	11.6
Total	104.3	90.1	104.3	90.1
Foreign government
Level 3 Financial Assets:
Balance, beginning of period	9.5	6.9	8.0	3.1
Purchases	0	0	0	3.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Transfers into Level 3	0	0	1.6	0
Transfers out of Level 3	(5.8)	0	(6.1)	0
Realized and unrealized gains (losses) included in income	0	0	0	0
Unrealized gains (losses) included in OCI	(0.4)	0.3	(0.2)	0.6
Total	3.3	7.2	3.3	7.2
Corporate
Level 3 Financial Assets:
Balance, beginning of period	903.2	577.8	801.5	513.8
Purchases	96.0	75.5	184.7	140.4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	3.4	6.1	5.1	6.7
Transfers into Level 3	30.4	24.6	31.9	37.8
Transfers out of Level 3	(2.1)	(8.1)	(10.9)	(19.1)
Realized and unrealized gains (losses) included in income	0	0	0	0
Unrealized gains (losses) included in OCI	(68.2)	0.8	(46.2)	(1.7)
Total	955.9	664.5	955.9	664.5
Commercial mortgage-backed (CMBS)
Level 3 Financial Assets:
Balance, beginning of period	18.7	20.4	19.3	35.4
Purchases	15.6	0	15.6	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Transfers into Level 3	0	1.0	0	1.0
Transfers out of Level 3	0	(3.7)	(1.5)	(18.0)
Realized and unrealized gains (losses) included in income	(0.1)	0	(0.1)	0
Unrealized gains (losses) included in OCI	(0.7)	0.1	0.2	(0.6)
Total	33.5	17.8	33.5	17.8
Residential mortgage-backed (RMBS)
Level 3 Financial Assets:
Balance, beginning of period	214.3	255.4	222.6	268.0
Purchases	0.3	0.4	0.6	0.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	10.5	10.1	14.2	12.0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Realized and unrealized gains (losses) included in income	0.1	(0.6)	(0.1)	(0.9)
Unrealized gains (losses) included in OCI	(1.7)	2.2	(6.4)	(8.7)
Total	202.5	247.3	202.5	247.3
CDO/CLO
Level 3 Financial Assets:
Balance, beginning of period	62.2	64.9	56.8	62.5
Purchases	11.8	0	22.7	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	5.6	2.2	4.8	1.7
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Realized and unrealized gains (losses) included in income	0	0	(0.3)	0
Unrealized gains (losses) included in OCI	(0.8)	(5.4)	(6.8)	(3.5)
Total	67.6	57.3	67.6	57.3
Other asset-backed
Level 3 Financial Assets:
Balance, beginning of period	79.9	78.7	80.1	81.2
Purchases	0.5	0	0.5	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	2.4	2.5	3.3	3.4
Transfers into Level 3	0	2.9	0	3.4
Transfers out of Level 3	0	(8.6)	0	(1.8)
Realized and unrealized gains (losses) included in income	0	0	0	0
Unrealized gains (losses) included in OCI	(4.1)	0.6	(3.4)	(8.3)
Total	73.9	71.1	73.9	71.1
Debt Securities
Level 3 Financial Assets:
Balance, beginning of period	1,417.4	1,109.5	1,330.3	1,044.4
Purchases	156.2	77.9	261.6	161.7
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	23.6	22.3	29.4	27.5
Transfers into Level 3	30.4	30.9	33.5	57.5
Transfers out of Level 3	(7.9)	(20.4)	(18.5)	(38.9)
Realized and unrealized gains (losses) included in income	0	(0.6)	(0.5)	(0.9)
Unrealized gains (losses) included in OCI	(84.4)	8.0	(88.9)	(13.3)
Total	1,488.1	1,183.0	1,488.1	1,183.0
Related party reinsurance derivative
Level 3 Financial Assets:
Balance, beginning of period	0	(1.8)	0	3.5
Purchases	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Realized and unrealized gains (losses) included in income	0	6.0	0	0.7
Unrealized gains (losses) included in OCI	0	0	0	0
Total	0	4.2	0	4.2
Fair value investments
Level 3 Financial Assets:
Balance, beginning of period	31.4	20.4	22.7	27.5
Purchases	4.8	2.9	14.6	2.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	1.0	1.7	1.5	2.5
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Realized and unrealized gains (losses) included in income	(0.8)	7.3	(1.4)	1.0
Unrealized gains (losses) included in OCI	0	0	0	0
Total	34.4	28.9	34.4	28.9
Total assets
Level 3 Financial Assets:
Balance, beginning of period	1,448.8	1,128.1	1,353.0	1,075.4
Purchases	161.0	80.8	276.2	164.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	24.6	24.0	30.9	30.0
Transfers into Level 3	30.4	30.9	33.5	57.5
Transfers out of Level 3	(7.9)	(20.4)	(18.5)	(38.9)
Realized and unrealized gains (losses) included in income	(0.8)	12.7	(1.9)	0.8
Unrealized gains (losses) included in OCI	(84.4)	8.0	(88.9)	(13.3)
Total	$ 1,522.5	$ 1,216.1	$ 1,522.5	$ 1,216.1