Fair Value Measurements - Level 3 Inputs Reconciliation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Level 1 to level 3 Transfers, Amount	$ 0	$ 0
Fair Value, Level 3 to level 1 Transfers, Amount	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	520.4	348.1
Included in Condensed Consolidated Statements of Operations	(1.3)	3.6
Included in Other Comprehensive Income	(4.9)	1.0
Purchases	78.3	61.3
Settlements	(51.2)	(20.2)
Sales	(0.3)	(4.0)
Transfers into Level 3	2.4	0.9
Transfers out of Level 3	(5.8)	(6.1)
Balance at End of Period	537.6	384.6
Fair Value, Level 1 to level 2 Transfers, Amount	0	0
Fair Value, Level 2 to Level 1 Transfers, Amount	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	361.0	235.1
Included in Condensed Consolidated Statements of Operations	(0.8)	0.1
Included in Other Comprehensive Income	(3.8)	0.9
Purchases	38.7	44.8
Settlements	(45.8)	(16.2)
Sales	0	(0.3)
Transfers into Level 3	0	0.9
Transfers out of Level 3	(5.8)	(6.1)
Balance at End of Period	343.5	259.2
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	4.7	6.1
Included in Condensed Consolidated Statements of Operations	(0.3)	0
Included in Other Comprehensive Income	0	0.2
Purchases	0	0
Settlements	0	0
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance at End of Period	4.4	6.3
Mortgage and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	0.1	0.3
Included in Condensed Consolidated Statements of Operations	0	0
Included in Other Comprehensive Income	(0.3)	0
Purchases	29.0	0
Settlements	0	0
Sales	0	0
Transfers into Level 3	2.4	0
Transfers out of Level 3	0	0
Balance at End of Period	31.2	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	13.3	13.5
Included in Condensed Consolidated Statements of Operations	0	3.2
Included in Other Comprehensive Income	1.5	(1.2)
Purchases	0.1	0.5
Settlements	0	0
Sales	(0.3)	(3.7)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance at End of Period	14.6	12.3
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	141.3	93.1
Included in Condensed Consolidated Statements of Operations	(0.2)	0.3
Included in Other Comprehensive Income	(2.3)	1.1
Purchases	10.5	16.0
Settlements	(5.4)	(4.0)
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance at End of Period	$ 143.9	$ 106.5