Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		$ 531,600,000	$ 606,800,000	$ 518,400,000
Included in Condensed Consolidated Statements of Operations		(1,500,000)	(10,700,000)	(5,500,000)
Included in Other Comprehensive Income		(500,000)	6,000,000	2,500,000
Purchases		115,700,000	109,600,000	156,500,000
Settlements		(10,800,000)	(14,900,000)	(15,000,000)
Sales		(46,900,000)	(50,000,000)	(65,500,000)
Transfers into Level 3	$ 0	800,000	0	800,000
Transfers out of Level 3	0	(16,900,000)	0	(20,700,000)
Balance at End of Period	646,800,000	571,500,000	646,800,000	571,500,000
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	431,800,000	374,800,000	436,300,000	360,600,000
Included in Condensed Consolidated Statements of Operations	(6,300,000)	(800,000)	(8,800,000)	(3,100,000)
Included in Other Comprehensive Income	4,100,000	(3,800,000)	3,300,000	(1,400,000)
Purchases	48,100,000	89,100,000	88,200,000	125,300,000
Settlements	(8,500,000)	(2,700,000)	(14,300,000)	(6,900,000)
Sales	(9,800,000)	(37,700,000)	(45,300,000)	(55,600,000)
Transfers into Level 3		0		0
Transfers out of Level 3		(12,300,000)		(12,300,000)
Balance at End of Period	459,400,000	406,600,000	459,400,000	406,600,000
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	3,300,000	6,400,000	3,800,000	6,700,000
Included in Condensed Consolidated Statements of Operations	0	(500,000)	0	(500,000)
Included in Other Comprehensive Income	(300,000)	(300,000)	(400,000)	(600,000)
Purchases	0	0	0	0
Settlements	(100,000)	(200,000)	(500,000)	(200,000)
Sales	0	0	0	0
Transfers into Level 3		0		0
Transfers out of Level 3		0		0
Balance at End of Period	2,900,000	5,400,000	2,900,000	5,400,000
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	83,200,000	69,900,000	87,300,000	64,400,000
Included in Condensed Consolidated Statements of Operations	300,000	300,000	(300,000)	400,000
Included in Other Comprehensive Income	3,000,000	(100,000)	(2,200,000)	1,200,000
Purchases	17,600,000	15,300,000	19,300,000	19,400,000
Settlements	0	(7,500,000)	0	(7,500,000)
Sales	(1,900,000)	(7,500,000)	(1,900,000)	(7,500,000)
Transfers into Level 3		0		0
Transfers out of Level 3		0		0
Balance at End of Period	102,200,000	70,400,000	102,200,000	70,400,000
Other Mortgage- and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	3,600,000	3,900,000	3,800,000	3,900,000
Included in Condensed Consolidated Statements of Operations	0	0	0	0
Included in Other Comprehensive Income	200,000	100,000	0	100,000
Purchases	0	1,300,000	0	1,300,000
Settlements	(100,000)	(100,000)	(100,000)	(100,000)
Sales	0	(1,200,000)	0	(1,200,000)
Transfers into Level 3		0		0
Transfers out of Level 3		0		0
Balance at End of Period	3,700,000	4,000,000	3,700,000	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	30,300,000	33,200,000	30,000,000	38,800,000
Included in Condensed Consolidated Statements of Operations	100,000	(400,000)	(900,000)	(1,200,000)
Included in Other Comprehensive Income	1,200,000	2,900,000	2,500,000	2,200,000
Purchases	100,000	6,600,000	100,000	7,000,000
Settlements	0	(300,000)	0	(300,000)
Sales	(300,000)	(500,000)	(300,000)	(1,200,000)
Transfers into Level 3		800,000		800,000
Transfers out of Level 3		(4,600,000)		(8,400,000)
Balance at End of Period	31,400,000	37,700,000	31,400,000	37,700,000
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	42,200,000	43,400,000	45,600,000	44,000,000
Included in Condensed Consolidated Statements of Operations	(300,000)	(100,000)	(700,000)	(1,100,000)
Included in Other Comprehensive Income	3,300,000	700,000	2,800,000	1,000,000
Purchases	2,000,000	3,400,000	2,000,000	3,500,000
Settlements	0	0	0	0
Sales	0	0	(2,500,000)	0
Transfers into Level 3		0		0
Transfers out of Level 3		0		0
Balance at End of Period	47,200,000	$ 47,400,000	47,200,000	$ 47,400,000
Fair Value Option Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	594,400,000
Included in Condensed Consolidated Statements of Operations	(6,200,000)
Included in Other Comprehensive Income	11,500,000
Purchases	67,800,000
Settlements	(8,700,000)
Sales	(12,000,000)
Balance at End of Period	$ 646,800,000		$ 646,800,000