Fair Value Measurements - Level 3 Inputs Reconciliation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 626.7	$ 520.4
Included in Condensed Consolidated Statements of Operations	0.4	(1.3)
Included in Other Comprehensive Income	13.3	(4.9)
Purchases	63.2	78.3
Settlements	(31.0)	(51.2)
Sales	(10.9)	(0.3)
Transfers into Level 3	0	2.4
Transfers out of Level 3	(0.6)	(5.8)
Balance at End of Period	661.1	537.6
Fair Value, Level 1 to level 2 Transfers, Amount	0	0
Fair Value, Level 2 to Level 1 Transfers, Amount	0	0
Fair Value, Level 1 to level 3 Transfers, Amount	0	0
Fair Value, Level 3 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	364.1	361.0
Included in Condensed Consolidated Statements of Operations	0.1	(0.8)
Included in Other Comprehensive Income	2.5	(3.8)
Purchases	48.3	38.7
Settlements	(24.8)	(45.8)
Sales	(10.2)	0
Transfers into Level 3		0
Transfers out of Level 3	(0.6)	(5.8)
Balance at End of Period	379.4	343.5
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	7.4	4.7
Included in Condensed Consolidated Statements of Operations	0	(0.3)
Included in Other Comprehensive Income	0.2	0
Purchases	0	0
Settlements	(0.1)	0
Sales	0	0
Transfers into Level 3		0
Transfers out of Level 3	0	0
Balance at End of Period	7.5	4.4
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	44.7	0
Included in Condensed Consolidated Statements of Operations	0	0
Included in Other Comprehensive Income	0.3	(0.1)
Purchases	0	24.5
Settlements	0	0
Sales	0	0
Transfers into Level 3		0
Transfers out of Level 3	0	0
Balance at End of Period	45.0	24.4
Mortgage and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	4.5	0.1
Included in Condensed Consolidated Statements of Operations	0.1	0
Included in Other Comprehensive Income	0	(0.2)
Purchases	0	4.5
Settlements	(0.8)	0
Sales	0	0
Transfers into Level 3		2.4
Transfers out of Level 3	0	0
Balance at End of Period	3.8	6.8
Preferred and Common Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	32.1	13.3
Included in Condensed Consolidated Statements of Operations	0	0
Included in Other Comprehensive Income	3.4	1.5
Purchases	2.6	0.1
Settlements	0	0
Sales	0	(0.3)
Transfers into Level 3		0
Transfers out of Level 3	0	0
Balance at End of Period	38.1	14.6
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	173.9	141.3
Included in Condensed Consolidated Statements of Operations	0.2	(0.2)
Included in Other Comprehensive Income	6.9	(2.3)
Purchases	12.3	10.5
Settlements	(5.3)	(5.4)
Sales	(0.7)	0
Transfers into Level 3		0
Transfers out of Level 3	0	0
Balance at End of Period	$ 187.3	$ 143.9