Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 884,000,000	$ 716,300,000	$ 713,900,000	$ 626,700,000
Included in Condensed Consolidated Statements of Operations	(1,400,000)	(400,000)	(4,200,000)	(200,000)
Included in Other Comprehensive Income	(5,200,000)	19,100,000	(4,000,000)	36,900,000
Purchases	90,000,000	67,400,000	377,300,000	227,100,000
Settlements	(42,200,000)	(50,800,000)	(76,500,000)	(119,000,000)
Sales	(7,400,000)	(33,200,000)	(72,900,000)	(52,500,000)
Transfers into Level 3	0	0	4,900,000	0
Transfers out of Level 3	0	0	(20,700,000)	(600,000)
Balance at End of Period	917,800,000	718,400,000	917,800,000	718,400,000
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	406,600,000	384,800,000	360,600,000	364,100,000
Included in Condensed Consolidated Statements of Operations	(400,000)	(1,800,000)	(3,500,000)	(4,100,000)
Included in Other Comprehensive Income	(2,000,000)	(1,400,000)	(3,400,000)	4,000,000
Purchases	59,400,000	31,700,000	184,700,000	117,600,000
Settlements	(28,300,000)	(41,600,000)	(35,200,000)	(92,400,000)
Sales	(6,800,000)	(30,700,000)	(62,400,000)	(47,600,000)
Transfers into Level 3			0
Transfers out of Level 3			(12,300,000)	(600,000)
Balance at End of Period	428,500,000	341,000,000	428,500,000	341,000,000
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	5,400,000	7,100,000	6,700,000	7,400,000
Included in Condensed Consolidated Statements of Operations	(800,000)	0	(1,300,000)	0
Included in Other Comprehensive Income	0	500,000	(600,000)	400,000
Purchases	0	0	0	0
Settlements	(100,000)	(200,000)	(300,000)	(400,000)
Sales	0	0	0	0
Transfers into Level 3			0
Transfers out of Level 3			0	0
Balance at End of Period	4,500,000	7,400,000	4,500,000	7,400,000
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	70,400,000	49,500,000	64,400,000	44,700,000
Included in Condensed Consolidated Statements of Operations	100,000	0	500,000	100,000
Included in Other Comprehensive Income	(1,500,000)	(300,000)	(300,000)	(400,000)
Purchases	5,300,000	13,300,000	24,700,000	18,100,000
Settlements	0	0	(7,500,000)	0
Sales	0	0	(7,500,000)	0
Transfers into Level 3			0
Transfers out of Level 3			0	0
Balance at End of Period	74,300,000	62,500,000	74,300,000	62,500,000
Other Mortgage- and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	4,000,000	3,900,000	3,900,000	4,500,000
Included in Condensed Consolidated Statements of Operations	0	0	0	100,000
Included in Other Comprehensive Income	(100,000)	100,000	0	200,000
Purchases	0	0	1,300,000	0
Settlements	0	0	(100,000)	(800,000)
Sales	0	0	(1,200,000)	0
Transfers into Level 3			0
Transfers out of Level 3			0	0
Balance at End of Period	3,900,000	4,000,000	3,900,000	4,000,000
Preferred and Common Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	37,700,000	36,300,000	38,800,000	32,100,000
Included in Condensed Consolidated Statements of Operations	0	800,000	(1,200,000)	3,100,000
Included in Other Comprehensive Income	(600,000)	1,000,000	1,600,000	3,000,000
Purchases	200,000	100,000	7,200,000	2,800,000
Settlements	(200,000)	(800,000)	(500,000)	(3,600,000)
Sales	(600,000)	(2,200,000)	(1,800,000)	(2,200,000)
Transfers into Level 3			800,000
Transfers out of Level 3			(8,400,000)	0
Balance at End of Period	36,500,000	35,200,000	36,500,000	35,200,000
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	199,900,000	194,400,000	186,200,000	173,900,000
Included in Condensed Consolidated Statements of Operations	600,000	300,000	(500,000)	0
Included in Other Comprehensive Income	(1,000,000)	19,200,000	(1,300,000)	29,700,000
Purchases	18,100,000	12,300,000	48,400,000	38,600,000
Settlements	(13,600,000)	(8,200,000)	(32,900,000)	(21,800,000)
Sales	0	(300,000)	0	(2,700,000)
Transfers into Level 3			4,100,000
Transfers out of Level 3			0	0
Balance at End of Period	204,000,000	217,700,000	204,000,000	217,700,000
Fair Value Option Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	160,000,000	40,300,000	53,300,000	0
Included in Condensed Consolidated Statements of Operations	(900,000)	300,000	1,800,000	600,000
Included in Other Comprehensive Income	0	0	0	0
Purchases	7,000,000	10,000,000	111,000,000	50,000,000
Settlements	0	0	0	0
Sales	0	0	0	0
Transfers into Level 3			0
Transfers out of Level 3			0	0
Balance at End of Period	$ 166,100,000	$ 50,600,000	$ 166,100,000	$ 50,600,000