Fair Value Measurements - Level 3 Inputs Reconciliation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 556.2	$ 404.4	$ 520.4	$ 348.1
Included in Condensed Consolidated Statements of Operations	(2.4)	1.4	(3.9)	7.0
Included in Other Comprehensive Income	1.1	8.3	(5.3)	5.3
Purchases	50.1	44.2	178.9	157.9
Settlements	(30.4)	(13.6)	(114.6)	(64.2)
Sales	(0.9)		(1.2)	(4.2)
Transfers into Level 3	3.0	5.1	8.2	6.0
Transfers out of Level 3	0	0	(5.8)	(6.1)
Balance at End of Period	576.7	449.8	576.7	449.8
Fair Value, Level 1 to level 2 Transfers, Amount	0	0	0	0
Fair Value, Level 2 to Level 1 Transfers, Amount	0	0	0	0
Fair Value, Level 1 to level 3 Transfers, Amount	0	0	0	0
Fair Value, Level 3 to level 1 Transfers, Amount	0	0	0	0
Corporate Bonds and Notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	351.5	276.3	361.0	235.1
Included in Condensed Consolidated Statements of Operations	(1.8)	1.4	(2.7)	3.5
Included in Other Comprehensive Income	0.9	1.5	(7.2)	2.2
Purchases	17.3	37.6	92.9	123.1
Settlements	(21.0)	(10.8)	(94.1)	(52.2)
Sales	(0.5)		(0.5)	(0.5)
Transfers into Level 3	3.0	5.1	5.8	6.0
Transfers out of Level 3			(5.8)	(6.1)
Balance at End of Period	349.4	311.1	349.4	311.1
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	4.3	4.6	4.7	6.1
Included in Condensed Consolidated Statements of Operations	0	0	(0.4)	0
Included in Other Comprehensive Income	(0.1)	0.2	(0.1)	0.3
Purchases	5.1	0	5.1	0
Settlements	(0.1)	0	(0.1)	(1.6)
Sales	0		0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3			0	0
Balance at End of Period	9.2	4.8	9.2	4.8
Mortgage and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	36.5	0.2	0.1	0.3
Included in Condensed Consolidated Statements of Operations	0	0	0.1	0
Included in Other Comprehensive Income	0.3	0	(0.3)	0
Purchases	11.2	0	45.8	0
Settlements	0	0	(0.1)	(0.1)
Sales	0		0	0
Transfers into Level 3	0	0	2.4	0
Transfers out of Level 3			0	0
Balance at End of Period	48.0	0.2	48.0	0.2
Preferred and Common Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	16.6	13.4	13.3	13.5
Included in Condensed Consolidated Statements of Operations	(0.9)	0	(1.0)	3.2
Included in Other Comprehensive Income	0.8	0.5	3.9	(0.3)
Purchases	0	0.1	0.6	1.3
Settlements	0	0	0	0
Sales	(0.4)		(0.7)	(3.7)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3			0	0
Balance at End of Period	16.1	14.0	16.1	14.0
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	147.3	109.9	141.3	93.1
Included in Condensed Consolidated Statements of Operations	0.3	0	0.1	0.3
Included in Other Comprehensive Income	(0.8)	6.1	(1.6)	3.1
Purchases	16.5	6.5	34.5	33.5
Settlements	(9.3)	(2.8)	(20.3)	(10.3)
Sales	0		0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3			0	0
Balance at End of Period	$ 154.0	$ 119.7	$ 154.0	$ 119.7