Fair Value Measurements - Level 3 Inputs Reconciliation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 716.3	$ 556.2	$ 626.7	$ 520.4
Included in Condensed Consolidated Statements of Operations	(0.4)	(2.4)	(0.2)	(3.9)
Included in Other Comprehensive Income	19.1	1.1	36.9	(5.3)
Purchases	67.4	50.1	227.1	178.9
Settlements	(50.8)	(30.4)	(119.0)	(114.6)
Sales	(33.2)	(0.9)	(52.5)	(1.2)
Transfers into Level 3	0	3.0	0	8.2
Transfers out of Level 3	0	0	(0.6)	(5.8)
Balance at End of Period	718.4	576.7	718.4	576.7
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	384.8	351.5	364.1	361.0
Included in Condensed Consolidated Statements of Operations	(1.8)	(1.8)	(4.1)	(2.7)
Included in Other Comprehensive Income	(1.4)	0.9	4.0	(7.2)
Purchases	31.7	17.3	117.6	92.9
Settlements	(41.6)	(21.0)	(92.4)	(94.1)
Sales	(30.7)	(0.5)	(47.6)	(0.5)
Transfers into Level 3		3.0		5.8
Transfers out of Level 3			(0.6)	(5.8)
Balance at End of Period	341.0	349.4	341.0	349.4
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	7.1	4.3	7.4	4.7
Included in Condensed Consolidated Statements of Operations	0	0	0	(0.4)
Included in Other Comprehensive Income	0.5	(0.1)	0.4	(0.1)
Purchases	0	5.1	0	5.1
Settlements	(0.2)	(0.1)	(0.4)	(0.1)
Sales	0	0	0	0
Transfers into Level 3		0		0
Transfers out of Level 3			0	0
Balance at End of Period	7.4	9.2	7.4	9.2
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	49.5	29.8	44.7	0
Included in Condensed Consolidated Statements of Operations	0	0	0.1	0.1
Included in Other Comprehensive Income	(0.3)	0.4	(0.4)	0
Purchases	13.3	11.1	18.1	41.2
Settlements	0	0	0	0
Sales	0	0	0	0
Transfers into Level 3		0		0
Transfers out of Level 3			0	0
Balance at End of Period	62.5	41.3	62.5	41.3
Mortgage and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	3.9	6.7	4.5	0.1
Included in Condensed Consolidated Statements of Operations	0	0	0.1	0
Included in Other Comprehensive Income	0.1	(0.1)	0.2	(0.3)
Purchases	0	0.1	0	4.6
Settlements	0	0	(0.8)	(0.1)
Sales	0	0	0	0
Transfers into Level 3		0		2.4
Transfers out of Level 3			0	0
Balance at End of Period	4.0	6.7	4.0	6.7
Preferred and Common Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	36.3	16.6	32.1	13.3
Included in Condensed Consolidated Statements of Operations	0.8	(0.9)	3.1	(1.0)
Included in Other Comprehensive Income	1.0	0.8	3.0	3.9
Purchases	0.1	0	2.8	0.6
Settlements	(0.8)	0	(3.6)	0
Sales	(2.2)	(0.4)	(2.2)	(0.7)
Transfers into Level 3		0		0
Transfers out of Level 3			0	0
Balance at End of Period	35.2	16.1	35.2	16.1
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	194.4	147.3	173.9	141.3
Included in Condensed Consolidated Statements of Operations	0.3	0.3	0	0.1
Included in Other Comprehensive Income	19.2	(0.8)	29.7	(1.6)
Purchases	12.3	16.5	38.6	34.5
Settlements	(8.2)	(9.3)	(21.8)	(20.3)
Sales	(0.3)	0	(2.7)	0
Transfers into Level 3		0		0
Transfers out of Level 3			0	0
Balance at End of Period	217.7	154.0	217.7	154.0
Fair Value Option Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	40.3		0
Included in Condensed Consolidated Statements of Operations	0.3		0.6
Included in Other Comprehensive Income	0		0
Purchases	10.0		50.0
Settlements	0		0
Sales	0		0
Transfers out of Level 3			0
Balance at End of Period	$ 50.6		$ 50.6