Fair Value of Financial Instruments - Reconciliation of Level 3 Financial Assets (Details) - Level 3 - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Level 3 Financial Assets:
Balance, beginning of period	$ 2,141.6	$ 1,707.4	$ 2,146.8	$ 1,695.8
Purchases	150.5	50.8	289.4	153.9
Sales	(62.4)	(50.3)	(91.5)	(68.8)
Transfers into Level 3	38.3	2.5	26.8	2.4
Transfers out of Level 3	(32.0)	(7.2)	(177.2)	(95.3)
Realized and unrealized gains (losses) included in income [1]	0.2	0.1	1.1	1.2
Unrealized gains (losses) included in OCI	(61.1)	24.9	(20.3)	39.0
Total	2,175.1	1,728.2	2,175.1	1,728.2
U.S. government and agency
Level 3 Financial Assets:
Balance, beginning of period	81.1	65.8	80.2	58.8
Purchases	0.0	0.0	7.9	7.9
Sales	(0.9)	(2.8)	(1.4)	(4.8)
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Realized and unrealized gains (losses) included in income [1]	0.0	0.0	0.0	0.0
Unrealized gains (losses) included in OCI	5.9	1.3	(0.6)	2.4
Total	86.1	64.3	86.1	64.3
State and political subdivision
Level 3 Financial Assets:
Balance, beginning of period	201.2	137.8	193.3	133.8
Purchases	10.7	4.6	17.0	5.9
Sales	(1.0)	(0.5)	(1.8)	(0.9)
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	(2.1)	0.0
Realized and unrealized gains (losses) included in income [1]	0.0	0.0	0.0	0.0
Unrealized gains (losses) included in OCI	(7.3)	2.7	(2.8)	5.8
Total	203.6	144.6	203.6	144.6
Foreign government
Level 3 Financial Assets:
Balance, beginning of period	10.5	3.4	16.1	3.3
Purchases	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Transfers into Level 3	0.0	0.0	0.3	0.0
Transfers out of Level 3	0.0	0.0	(6.5)	0.0
Realized and unrealized gains (losses) included in income [1]	0.0	0.0	0.0	0.0
Unrealized gains (losses) included in OCI	0.2	0.1	0.8	0.2
Total	10.7	3.5	10.7	3.5
Corporate
Level 3 Financial Assets:
Balance, beginning of period	1,452.5	1,122.3	1,417.2	1,091.9
Purchases	85.9	23.9	193.2	104.6
Sales	(40.5)	(20.2)	(56.5)	(31.3)
Transfers into Level 3	38.3	2.5	26.5	2.4
Transfers out of Level 3	(16.2)	(2.7)	(103.2)	(65.5)
Realized and unrealized gains (losses) included in income [1]	0.1	0.0	0.3	0.0
Unrealized gains (losses) included in OCI	(56.8)	7.4	(14.2)	31.1
Total	1,463.3	1,133.2	1,463.3	1,133.2
Commercial mortgage-backed (“CMBS”)
Level 3 Financial Assets:
Balance, beginning of period	6.9	6.4	64.6	31.9
Purchases	4.1	0.0	4.1	0.0
Sales	(0.1)	(3.6)	(0.2)	(3.9)
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	(2.0)	0.0	(58.3)	(25.5)
Realized and unrealized gains (losses) included in income [1]	0.0	0.0	0.0	0.0
Unrealized gains (losses) included in OCI	0.0	0.1	(1.3)	0.4
Total	8.9	2.9	8.9	2.9
Residential mortgage-backed (“RMBS”)
Level 3 Financial Assets:
Balance, beginning of period	158.3	171.4	151.6	176.1
Purchases	0.2	0.3	0.5	0.6
Sales	(10.3)	(5.8)	(15.3)	(10.5)
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	(12.2)	(4.5)	0.0	(4.3)
Realized and unrealized gains (losses) included in income [1]	0.0	(0.1)	(0.1)	0.2
Unrealized gains (losses) included in OCI	(0.6)	0.3	(1.3)	(0.5)
Total	135.4	161.6	135.4	161.6
Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”)
Level 3 Financial Assets:
Balance, beginning of period	96.5	86.2	83.5	70.9
Purchases	43.9	22.0	60.9	34.9
Sales	(0.5)	(11.9)	(5.8)	(8.6)
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Realized and unrealized gains (losses) included in income [1]	0.0	0.0	0.2	0.5
Unrealized gains (losses) included in OCI	(0.5)	0.6	0.6	(0.8)
Total	139.4	96.9	139.4	96.9
Other asset-backed (“ABS”)
Level 3 Financial Assets:
Balance, beginning of period	71.2	78.9	77.9	82.3
Purchases	4.5	0.0	4.5	0.0
Sales	(2.0)	(4.7)	(2.9)	(7.4)
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	(1.6)	0.0	(7.1)	0.0
Realized and unrealized gains (losses) included in income [1]	0.0	0.0	0.0	0.1
Unrealized gains (losses) included in OCI	(1.4)	0.5	(1.7)	(0.3)
Total	70.7	74.7	70.7	74.7
Debt securities
Level 3 Financial Assets:
Balance, beginning of period	2,078.2	1,672.2	2,084.4	1,649.0
Purchases	149.3	50.8	288.1	153.9
Sales	(55.3)	(49.5)	(83.9)	(67.4)
Transfers into Level 3	38.3	2.5	26.8	2.4
Transfers out of Level 3	(32.0)	(7.2)	(177.2)	(95.3)
Realized and unrealized gains (losses) included in income [1]	0.1	(0.1)	0.4	0.8
Unrealized gains (losses) included in OCI	(60.5)	13.0	(20.5)	38.3
Total	2,118.1	1,681.7	2,118.1	1,681.7
Equity securities
Level 3 Financial Assets:
Balance, beginning of period	29.5	0.0	28.7	11.2
Purchases	1.2	0.0	1.2	0.0
Sales	(2.5)	0.0	(2.5)	0.0
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Realized and unrealized gains (losses) included in income [1]	0.0	0.0	0.0	0.0
Unrealized gains (losses) included in OCI	(0.6)	11.9	0.2	0.7
Total	27.6	11.9	27.6	11.9
Fair value investments
Level 3 Financial Assets:
Balance, beginning of period	33.9	35.2	33.7	35.6
Purchases	0.0	0.0	0.1	0.0
Sales	(4.6)	(0.8)	(5.1)	(1.4)
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Realized and unrealized gains (losses) included in income [1]	0.1	0.2	0.7	0.4
Unrealized gains (losses) included in OCI	0.0	0.0	0.0	0.0
Total	$ 29.4	$ 34.6	$ 29.4	$ 34.6