Fair Value Measurements - Level 3 Inputs Reconciliation (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Level 1 to Level 2 Activity Transfers, Amount	$ 0	$ 2,200,000
Fair Value, Level 2 to Level 1 Activity Transfers, Amount	0	2,200,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	348,100,000	241,000,000
Included in Consolidated Statement of Income	6,800,000	8,100,000
Included in Other Comprehensive Income	8,900,000	(4,300,000)
Purchases	253,400,000	174,400,000
Settlements	(87,000,000)	(70,300,000)
Sales	(4,600,000)	(6,200,000)
Transfers Into Level 3	900,000	5,400,000
Transfers out of Level 3	(6,100,000)	0
Balance at December 31, 2012	520,400,000	348,100,000
Corporate Bonds and Notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	235,100,000	146,200,000
Included in Consolidated Statement of Income	4,000,000	(700,000)
Included in Other Comprehensive Income	1,400,000	3,400,000
Purchases	199,700,000	131,600,000
Settlements	(73,100,000)	(50,300,000)
Sales	(900,000)	(500,000)
Transfers Into Level 3	900,000	5,400,000
Transfers out of Level 3	(6,100,000)
Balance at December 31, 2012	361,000,000	235,100,000
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	6,100,000	4,500,000
Included in Consolidated Statement of Income	(400,000)	0
Included in Other Comprehensive Income	500,000	1,600,000
Purchases	100,000	0
Settlements	(1,600,000)	0
Sales	0	0
Transfers Into Level 3	0.0	0
Transfers out of Level 3	0.0
Balance at December 31, 2012	4,700,000	6,100,000
Mortgage and Asset Backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	300,000	400,000
Included in Consolidated Statement of Income	0	0
Included in Other Comprehensive Income	0	0
Purchases	0	0
Settlements	(200,000)	(100,000)
Sales	0	0
Transfers Into Level 3	0	0
Transfers out of Level 3	0
Balance at December 31, 2012	100,000	300,000
Preferred and Common Stock [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	13,500,000	14,700,000
Included in Consolidated Statement of Income	3,200,000	4,500,000
Included in Other Comprehensive Income	(1,300,000)	(3,100,000)
Purchases	1,600,000	3,500,000
Settlements	0	(400,000)
Sales	(3,700,000)	(5,700,000)
Transfers Into Level 3	0	0
Transfers out of Level 3	0
Balance at December 31, 2012	13,300,000	13,500,000
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	93,100,000	75,200,000
Included in Consolidated Statement of Income	0	4,300,000
Included in Other Comprehensive Income	8,300,000	(6,200,000)
Purchases	52,000,000	39,300,000
Settlements	(12,100,000)	(19,500,000)
Sales	0	0
Transfers Into Level 3	0	0
Transfers out of Level 3	0
Balance at December 31, 2012	141,300,000	93,100,000
Fair Value, Inputs, Level 3 [Member] | Private Equity Funds and Mezzanine Debt Funds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Unfunded Commitments	$ 86,200,000