Fair Value Measurements - Level 3 Inputs Reconciliation (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Level 1 to Level 2 Activity Transfers, Amount	$ 0	$ 0
Fair Value, Level 2 to Level 1 Activity Transfers, Amount	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	520.4	348.1
Included in Consolidated Statement of Income	(10.0)	6.8
Included in Other Comprehensive Income	1.0	8.9
Purchases	270.1	253.4
Settlements	(151.4)	(87.0)
Sales	(2.8)	(4.6)
Transfers Into Level 3	8.2	0.9
Transfers out of Level 3	(8.8)	(6.1)
Balance at December 31, 2012	626.7	520.4
Corporate Bonds and Notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	361.0	235.1
Included in Consolidated Statement of Income	(7.9)	4.0
Included in Other Comprehensive Income	(7.8)	1.4
Purchases	145.1	199.7
Settlements	(123.2)	(73.1)
Sales	(0.1)	(0.9)
Transfers Into Level 3	5.8	0.9
Transfers out of Level 3	(8.8)	(6.1)
Balance at December 31, 2012	364.1	361.0
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	4.7	6.1
Included in Consolidated Statement of Income	0.2	(0.4)
Included in Other Comprehensive Income	(1.3)	0.5
Purchases	5.1	0.1
Settlements	(1.3)	(1.6)
Sales	0	0
Transfers Into Level 3	0	0
Transfers out of Level 3	0	0
Balance at December 31, 2012	7.4	4.7
Mortgage and Asset Backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	0.1	0.3
Included in Consolidated Statement of Income	0.3	0
Included in Other Comprehensive Income	(0.1)	0
Purchases	48.6	0
Settlements	(0.1)	(0.2)
Sales	(2.0)	0
Transfers Into Level 3	2.4	0
Transfers out of Level 3	0	0
Balance at December 31, 2012	49.2	0.1
Preferred and Common Stock [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	13.3	13.5
Included in Consolidated Statement of Income	(1.0)	3.2
Included in Other Comprehensive Income	4.6	(1.3)
Purchases	15.9	1.6
Settlements	0	0
Sales	(0.7)	(3.7)
Transfers Into Level 3	0	0
Transfers out of Level 3	0	0
Balance at December 31, 2012	32.1	13.3
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	141.3	93.1
Included in Consolidated Statement of Income	(1.6)	0
Included in Other Comprehensive Income	5.6	8.3
Purchases	55.4	52.0
Settlements	(26.8)	(12.1)
Sales	0	0
Transfers Into Level 3	0	0
Transfers out of Level 3	0	0
Balance at December 31, 2012	$ 173.9	$ 141.3