Fair Value Measurements - Level 3 Inputs Reconciliation (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 713,900,000	$ 626,700,000
Included in Condensed Consolidated Statements of Operations	(3,100,000)	400,000
Included in Other Comprehensive Income	1,500,000	13,300,000
Purchases	51,900,000	63,200,000
Settlements	(13,800,000)	(31,000,000)
Sales	(18,600,000)	(10,900,000)
Transfers into Level 3	4,100,000	0
Transfers out of Level 3	(3,800,000)	(600,000)
Balance at End of Period	732,100,000	661,100,000
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	360,600,000	364,100,000
Included in Condensed Consolidated Statements of Operations	(2,300,000)	100,000
Included in Other Comprehensive Income	2,400,000	2,500,000
Purchases	36,200,000	48,300,000
Settlements	(4,200,000)	(24,800,000)
Sales	(17,900,000)	(10,200,000)
Transfers into Level 3	0
Transfers out of Level 3	0	(600,000)
Balance at End of Period	374,800,000	379,400,000
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	6,700,000	7,400,000
Included in Condensed Consolidated Statements of Operations	0	0
Included in Other Comprehensive Income	(300,000)	200,000
Purchases	0	0
Settlements	0	(100,000)
Sales	0	0
Transfers into Level 3	0
Transfers out of Level 3	0	0
Balance at End of Period	6,400,000	7,500,000
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	64,400,000	44,700,000
Included in Condensed Consolidated Statements of Operations	100,000	0
Included in Other Comprehensive Income	1,300,000	300,000
Purchases	4,100,000	0
Settlements	0	0
Sales	0	0
Transfers into Level 3	0
Transfers out of Level 3	0	0
Balance at End of Period	69,900,000	45,000,000
Mortgage and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	3,900,000	4,500,000
Included in Condensed Consolidated Statements of Operations	0	100,000
Included in Other Comprehensive Income	0	0
Purchases	0	0
Settlements	0	(800,000)
Sales	0	0
Transfers into Level 3	0
Transfers out of Level 3	0	0
Balance at End of Period	3,900,000	3,800,000
Preferred and Common Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	38,800,000	32,100,000
Included in Condensed Consolidated Statements of Operations	(800,000)	0
Included in Other Comprehensive Income	(700,000)	3,400,000
Purchases	400,000	2,600,000
Settlements	0	0
Sales	(700,000)	0
Transfers into Level 3	0
Transfers out of Level 3	(3,800,000)	0
Balance at End of Period	33,200,000	38,100,000
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	186,200,000	173,900,000
Included in Condensed Consolidated Statements of Operations	(1,000,000)	200,000
Included in Other Comprehensive Income	(1,200,000)	6,900,000
Purchases	11,200,000	12,300,000
Settlements	(9,600,000)	(5,300,000)
Sales	0	(700,000)
Transfers into Level 3	4,100,000
Transfers out of Level 3	0	0
Balance at End of Period	189,700,000	187,300,000
Fair Value Option Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	53,300,000
Included in Condensed Consolidated Statements of Operations	900,000
Included in Other Comprehensive Income	0
Purchases	0
Settlements	0
Sales	0
Transfers into Level 3	0
Transfers out of Level 3	0
Balance at End of Period	$ 54,200,000