Fair Value of Financial Instruments - Reconciliation of Level 3 Financial Assets (Details) (Fair Value, Measurements, Nonrecurring, Level 3, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
U.S. government and agency
Level 3 Financial Assets:
Balance, beginning of period	$ 47.1	$ 27.7	$ 25.9	$ 31.7
Purchases	16.4	0.7	40.0	2.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(3.1)	(0.1)	(3.3)	(3.7)
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 [1]	0	0	0	0
Unrealized gains (losses) included in OCI	(1.3)	0.3	(3.5)	(1.9)
Total	59.1	28.6	59.1	28.6
State and political subdivision
Level 3 Financial Assets:
Balance, beginning of period	104.3	90.1	116.1	48.7
Purchases	21.5	5.1	35.3	20.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(0.3)	(0.4)	(0.9)	(0.8)
Transfers into Level 3	0	0	0	12.9
Transfers out of Level 3	0	0	0	0
Realized and unrealized gains (losses) included in income [1]	0	0	0	0
Unrealized gains (losses) included in OCI	(1.1)	(1.4)	(26.1)	12.6
Total	124.4	93.4	124.4	93.4
Foreign government
Level 3 Financial Assets:
Balance, beginning of period	3.3	7.2	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	0	0	(6.1)	0
Realized and unrealized gains (losses) included in income [1]	0	0	0	0
Unrealized gains (losses) included in OCI	0	0.6	(0.2)	1.2
Total	3.3	7.8	3.3	7.8
Corporate
Level 3 Financial Assets:
Balance, beginning of period	955.9	664.5	801.5	513.8
Purchases	62.8	60.8	250.5	218.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(5.8)	(2.1)	(10.8)	(10.3)
Transfers into Level 3	17.5	0	40.9	20.0
Transfers out of Level 3	(6.3)	(6.1)	(10.9)	(14.2)
Realized and unrealized gains (losses) included in income [1]	0	0	0	0
Unrealized gains (losses) included in OCI	(8.9)	15.6	(56.0)	4.5
Total	1,015.2	732.7	1,015.2	732.7
Commercial mortgage-backed (“CMBS”)
Level 3 Financial Assets:
Balance, beginning of period	33.5	17.8	19.3	35.4
Purchases	8.1	0	23.7	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(0.3)	0	(0.3)	(0.5)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	(1.5)	(18.0)
Realized and unrealized gains (losses) included in income [1]	0	0	(0.1)	0
Unrealized gains (losses) included in OCI	(0.1)	0.2	0.1	1.1
Total	41.2	18.0	41.2	18.0
Residential mortgage-backed (“RMBS”)
Level 3 Financial Assets:
Balance, beginning of period	202.5	247.3	222.6	268.0
Purchases	0.3	1.7	1.0	1.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(8.6)	(7.5)	(19.4)	(26.9)
Transfers into Level 3	4.7	0	5.1	0
Transfers out of Level 3	0	0	0	0
Realized and unrealized gains (losses) included in income [1]	(0.1)	(0.4)	(0.3)	(0.9)
Unrealized gains (losses) included in OCI	(8.4)	4.5	(18.6)	4.1
Total	190.4	245.6	190.4	245.6
CDO/CLO
Level 3 Financial Assets:
Balance, beginning of period	67.6	57.3	56.8	62.5
Purchases	8.4	0	31.1	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(2.8)	(1.9)	(5.7)	(3.5)
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 [1]	0	0.2	(0.3)	0.2
Unrealized gains (losses) included in OCI	(6.5)	3.3	(15.2)	(0.3)
Total	66.7	58.9	66.7	58.9
Other asset-backed
Level 3 Financial Assets:
Balance, beginning of period	73.9	71.1	80.1	81.2
Purchases	12.8	3.5	13.4	9.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(3.6)	(3.2)	(4.7)	(5.9)
Transfers into Level 3	0	13.2	0	0
Transfers out of Level 3	0	0	0	(1.8)
Realized and unrealized gains (losses) included in income [1]	(0.1)	0	0	0
Unrealized gains (losses) included in OCI	(1.2)	(1.2)	(7.0)	0.4
Total	81.8	83.4	81.8	83.4
Debt Securities
Level 3 Financial Assets:
Balance, beginning of period	1,488.1	1,183.0	1,330.3	1,044.4
Purchases	130.3	71.8	395.0	255.7
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(24.5)	(15.2)	(45.1)	(51.6)
Transfers into Level 3	22.2	13.2	47.6	32.9
Transfers out of Level 3	(6.3)	(6.1)	(18.5)	(34.0)
Realized and unrealized gains (losses) included in income [1]	(0.2)	(0.2)	(0.7)	(0.7)
Unrealized gains (losses) included in OCI	(27.5)	21.9	(126.5)	21.7
Total	1,582.1	1,268.4	1,582.1	1,268.4
Related party reinsurance derivative
Level 3 Financial Assets:
Balance, beginning of period	0	4.2	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 [1]	0	(4.2)	0	(3.5)
Unrealized gains (losses) included in OCI	0	0	0	0
Total	0	0	0	0
Fair value investments
Level 3 Financial Assets:
Balance, beginning of period	34.4	28.9	22.7	27.5
Purchases	6.5	1.9	21.1	2.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(1.3)	(2.6)	(2.5)	(3.7)
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 [1]	(0.2)	0.1	(1.9)	1.6
Unrealized gains (losses) included in OCI	0	0	0	0
Total	39.4	28.3	39.4	28.3
Total assets
Level 3 Financial Assets:
Balance, beginning of period	1,522.5	1,216.1	1,353.0	1,075.4
Purchases	136.8	73.7	416.1	258.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(25.8)	(17.8)	(47.6)	(55.3)
Transfers into Level 3	22.2	13.2	47.6	32.9
Transfers out of Level 3	(6.3)	(6.1)	(18.5)	(34.0)
Realized and unrealized gains (losses) included in income [1]	(0.4)	(4.3)	(2.6)	(2.6)
Unrealized gains (losses) included in OCI	(27.5)	21.9	(126.5)	21.7
Total	$ 1,621.5	$ 1,296.7	$ 1,621.5	$ 1,296.7