Fair Value Measurements - Level 3 Inputs Reconciliation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 661.1	$ 537.6	$ 626.7	$ 520.4
Included in Condensed Consolidated Statements of Operations	(0.2)	(0.2)	0.2	(1.5)
Included in Other Comprehensive Income	4.5	(1.5)	17.8	(6.4)
Purchases	96.5	50.5	159.7	128.8
Settlements	(37.2)	(33.0)	(68.2)	(84.2)
Sales	(8.4)		(19.3)	(0.3)
Transfers into Level 3	0	2.8	0	5.2
Transfers out of Level 3	0	0	(0.6)	(5.8)
Balance at End of Period	716.3	556.2	716.3	556.2
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	379.4	343.5	364.1	361.0
Included in Condensed Consolidated Statements of Operations	(2.4)	(0.1)	(2.3)	(0.9)
Included in Other Comprehensive Income	2.9	(4.3)	5.4	(8.1)
Purchases	37.6	36.9	85.9	75.6
Settlements	(26.0)	(27.3)	(50.8)	(73.1)
Sales	(6.7)		(16.9)	0
Transfers into Level 3		2.8		2.8
Transfers out of Level 3			(0.6)	(5.8)
Balance at End of Period	384.8	351.5	384.8	351.5
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	7.5	4.4	7.4	4.7
Included in Condensed Consolidated Statements of Operations	0	(0.1)	0	(0.4)
Included in Other Comprehensive Income	(0.3)	0	(0.1)	0
Purchases	0	0	0	0
Settlements	(0.1)	0	(0.2)	0
Sales	0		0	0
Transfers into Level 3		0		0
Transfers out of Level 3			0	0
Balance at End of Period	7.1	4.3	7.1	4.3
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	45.0	24.4	44.7	0
Included in Condensed Consolidated Statements of Operations	0.1	0.1	0.1	0.1
Included in Other Comprehensive Income	(0.4)	(0.3)	(0.1)	(0.4)
Purchases	4.8	5.6	4.8	30.1
Settlements	0	0	0	0
Sales	0		0	0
Transfers into Level 3		0		0
Transfers out of Level 3			0	0
Balance at End of Period	49.5	29.8	49.5	29.8
Mortgage and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	3.8	6.8	4.5	0.1
Included in Condensed Consolidated Statements of Operations	0	0	0.1	0
Included in Other Comprehensive Income	0.1	0	0.1	(0.2)
Purchases	0	0	0	4.5
Settlements	0	(0.1)	(0.8)	(0.1)
Sales	0		0	0
Transfers into Level 3		0		2.4
Transfers out of Level 3			0	0
Balance at End of Period	3.9	6.7	3.9	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	38.1	14.6	32.1	13.3
Included in Condensed Consolidated Statements of Operations	2.3	(0.1)	2.3	(0.1)
Included in Other Comprehensive Income	(1.4)	1.6	2.0	3.1
Purchases	0.1	0.5	2.7	0.6
Settlements	(2.8)	0	(2.8)	0
Sales	0		0	(0.3)
Transfers into Level 3		0		0
Transfers out of Level 3			0	0
Balance at End of Period	36.3	16.6	36.3	16.6
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	187.3	143.9	173.9	141.3
Included in Condensed Consolidated Statements of Operations	(0.5)	0	(0.3)	(0.2)
Included in Other Comprehensive Income	3.6	1.5	10.5	(0.8)
Purchases	14.0	7.5	26.3	18.0
Settlements	(8.3)	(5.6)	(13.6)	(11.0)
Sales	(1.7)		(2.4)	0
Transfers into Level 3		0		0
Transfers out of Level 3			0	0
Balance at End of Period	194.4	147.3	194.4	147.3
Fair Value Option Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	0		0
Included in Condensed Consolidated Statements of Operations	0.3		0.3
Included in Other Comprehensive Income	0		0
Purchases	40.0		40.0
Settlements	0		0
Sales	0		0
Transfers out of Level 3			0
Balance at End of Period	$ 40.3		$ 40.3