Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 732,100,000	$ 661,100,000	$ 713,900,000	$ 626,700,000
Included in Condensed Consolidated Statements of Operations	300,000	(200,000)	(2,800,000)	200,000
Included in Other Comprehensive Income	(300,000)	4,500,000	1,200,000	17,800,000
Purchases	235,400,000	96,500,000	287,300,000	159,700,000
Settlements	(20,500,000)	(37,200,000)	(34,300,000)	(68,200,000)
Sales	(46,900,000)	(8,400,000)	(65,500,000)	(19,300,000)
Transfers into Level 3	800,000	0	4,900,000	0
Transfers out of Level 3	(16,900,000)	0	(20,700,000)	(600,000)
Balance at End of Period	884,000,000	716,300,000	884,000,000	716,300,000
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	374,800,000	379,400,000	360,600,000	364,100,000
Included in Condensed Consolidated Statements of Operations	(800,000)	(2,400,000)	(3,100,000)	(2,300,000)
Included in Other Comprehensive Income	(3,800,000)	2,900,000	(1,400,000)	5,400,000
Purchases	89,100,000	37,600,000	125,300,000	85,900,000
Settlements	(2,700,000)	(26,000,000)	(6,900,000)	(50,800,000)
Sales	(37,700,000)	(6,700,000)	(55,600,000)	(16,900,000)
Transfers into Level 3	0		0
Transfers out of Level 3	(12,300,000)		(12,300,000)	(600,000)
Balance at End of Period	406,600,000	384,800,000	406,600,000	384,800,000
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	6,400,000	7,500,000	6,700,000	7,400,000
Included in Condensed Consolidated Statements of Operations	(500,000)	0	(500,000)	0
Included in Other Comprehensive Income	(300,000)	(300,000)	(600,000)	(100,000)
Purchases	0	0	0	0
Settlements	(200,000)	(100,000)	(200,000)	(200,000)
Sales	0	0	0	0
Transfers into Level 3	0		0
Transfers out of Level 3	0		0	0
Balance at End of Period	5,400,000	7,100,000	5,400,000	7,100,000
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	69,900,000	45,000,000	64,400,000	44,700,000
Included in Condensed Consolidated Statements of Operations	300,000	100,000	400,000	100,000
Included in Other Comprehensive Income	(100,000)	(400,000)	1,200,000	(100,000)
Purchases	15,300,000	4,800,000	19,400,000	4,800,000
Settlements	(7,500,000)	0	(7,500,000)	0
Sales	(7,500,000)	0	(7,500,000)	0
Transfers into Level 3	0		0
Transfers out of Level 3	0		0	0
Balance at End of Period	70,400,000	49,500,000	70,400,000	49,500,000
Other Mortgage- and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	3,900,000	3,800,000	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	100,000	100,000
Purchases	1,300,000	0	1,300,000	0
Settlements	(100,000)	0	(100,000)	(800,000)
Sales	(1,200,000)	0	(1,200,000)	0
Transfers into Level 3	0		0
Transfers out of Level 3	0		0	0
Balance at End of Period	4,000,000	3,900,000	4,000,000	3,900,000
Preferred and Common Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	33,200,000	38,100,000	38,800,000	32,100,000
Included in Condensed Consolidated Statements of Operations	(400,000)	2,300,000	(1,200,000)	2,300,000
Included in Other Comprehensive Income	2,900,000	(1,400,000)	2,200,000	2,000,000
Purchases	6,600,000	100,000	7,000,000	2,700,000
Settlements	(300,000)	(2,800,000)	(300,000)	(2,800,000)
Sales	(500,000)	0	(1,200,000)	0
Transfers into Level 3	800,000		800,000
Transfers out of Level 3	(4,600,000)		(8,400,000)	0
Balance at End of Period	37,700,000	36,300,000	37,700,000	36,300,000
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	189,700,000	187,300,000	186,200,000	173,900,000
Included in Condensed Consolidated Statements of Operations	(100,000)	(500,000)	(1,100,000)	(300,000)
Included in Other Comprehensive Income	900,000	3,600,000	(300,000)	10,500,000
Purchases	19,100,000	14,000,000	30,300,000	26,300,000
Settlements	(9,700,000)	(8,300,000)	(19,300,000)	(13,600,000)
Sales	0	(1,700,000)	0	(2,400,000)
Transfers into Level 3	0		4,100,000
Transfers out of Level 3	0		0	0
Balance at End of Period	199,900,000	194,400,000	199,900,000	194,400,000
Fair Value Option Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	54,200,000	0	53,300,000	0
Included in Condensed Consolidated Statements of Operations	1,800,000	300,000	2,700,000	300,000
Included in Other Comprehensive Income	0	0	0	0
Purchases	104,000,000	40,000,000	104,000,000	40,000,000
Settlements	0	0	0	0
Sales	0	0	0	0
Transfers into Level 3	0		0
Transfers out of Level 3	0		0	0
Balance at End of Period	$ 160,000,000	$ 40,300,000	$ 160,000,000	$ 40,300,000