Fair Value Measurements - Level 3 Inputs Reconciliation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 404.4	$ 270.1	$ 348.1	$ 241.0
Included in Condensed Consolidated Statements of Operations	1.4	0.8	7.0	5.9
Included in Other Comprehensive Income	8.3	(14.7)	5.3	(19.6)
Purchases	44.2	29.4	157.9	101.5
Settlements	(13.6)	(18.2)	(64.2)	(55.2)
Sales	0		(4.2)	(6.2)
Transfers into Level 3	5.1	5.4	6.0	5.4
Transfers out of Level 3	0	0	6.1	0
Balance at End of Period	449.8	272.8	449.8	272.8
Fair Value, Level 1 to level 2 Transfers, Amount	0	0	0	2.2
Fair Value, Level 2 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	276.3	174.8	235.1	146.2
Included in Condensed Consolidated Statements of Operations	1.4	0.6	3.5	1.0
Included in Other Comprehensive Income	1.5	1.1	2.2	2.5
Purchases	37.6	20.1	123.1	76.1
Settlements	(10.8)	(15.4)	(52.2)	(44.1)
Sales	0		(0.5)	(0.5)
Transfers into Level 3	5.1		6.0	5.4
Transfers out of Level 3			6.1
Balance at End of Period	311.1	186.6	311.1	186.6
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	4.6	6.4	6.1	4.5
Included in Condensed Consolidated Statements of Operations	0	0	0	0
Included in Other Comprehensive Income	0.2	(1.9)	0.3	0
Purchases	0	0	0	0
Settlements	0	0	(1.6)	0
Sales	0		0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3			0
Balance at End of Period	4.8	4.5	4.8	4.5
Mortgage and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	0.2	0.3	0.3	0.4
Included in Condensed Consolidated Statements of Operations	0	0	0	0
Included in Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	0
Settlements	0	0	(0.1)	(0.1)
Sales	0		0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3			0
Balance at End of Period	0.2	0.3	0.2	0.3
Preferred and Common Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	13.4	8.5	13.5	14.7
Included in Condensed Consolidated Statements of Operations	0	(0.1)	3.2	4.6
Included in Other Comprehensive Income	0.5	(0.2)	(0.3)	(5.5)
Purchases	0.1	1.0	1.3	1.5
Settlements	0	0	0	(0.4)
Sales	0		(3.7)	(5.7)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3			0
Balance at End of Period	14.0	9.2	14.0	9.2
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	109.9	80.1	93.1	75.2
Included in Condensed Consolidated Statements of Operations	0	0.3	0.3	0.3
Included in Other Comprehensive Income	6.1	(13.7)	3.1	(16.6)
Purchases	6.5	8.3	33.5	23.9
Settlements	(2.8)	(2.8)	(10.3)	(10.6)
Sales	0		0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3			0
Balance at End of Period	$ 119.7	$ 72.2	$ 119.7	$ 72.2