Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 606,800,000	$ 518,400,000
Included in Condensed Consolidated Statements of Operations	(4,500,000)	(4,000,000)
Included in Other Comprehensive Income	(5,500,000)	3,000,000
Purchases	41,800,000	40,800,000
Settlements	(6,200,000)	(4,200,000)
Sales	(38,000,000)	(18,600,000)
Transfers into Level 3	0	0
Transfers out of Level 3	0	(3,800,000)
Balance at End of Period	594,400,000	531,600,000
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	436,300,000	360,600,000
Included in Condensed Consolidated Statements of Operations	(2,500,000)	(2,300,000)
Included in Other Comprehensive Income	(800,000)	2,400,000
Purchases	40,100,000	36,200,000
Settlements	(5,800,000)	(4,200,000)
Sales	(35,500,000)	(17,900,000)
Transfers out of Level 3		0
Balance at End of Period	431,800,000	374,800,000
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	3,800,000	6,700,000
Included in Condensed Consolidated Statements of Operations	0	0
Included in Other Comprehensive Income	(100,000)	(300,000)
Purchases	0	0
Settlements	(400,000)	0
Sales	0	0
Transfers out of Level 3		0
Balance at End of Period	3,300,000	6,400,000
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	87,300,000	64,400,000
Included in Condensed Consolidated Statements of Operations	(600,000)	100,000
Included in Other Comprehensive Income	(5,200,000)	1,300,000
Purchases	1,700,000	4,100,000
Settlements	0	0
Sales	0	0
Transfers out of Level 3		0
Balance at End of Period	83,200,000	69,900,000
Other Mortgage- and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	3,800,000	3,900,000
Included in Condensed Consolidated Statements of Operations	0	0
Included in Other Comprehensive Income	(200,000)	0
Purchases	0	0
Settlements	0	0
Sales	0	0
Transfers out of Level 3		0
Balance at End of Period	3,600,000	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	30,000,000	38,800,000
Included in Condensed Consolidated Statements of Operations	(1,000,000)	(800,000)
Included in Other Comprehensive Income	1,300,000	(700,000)
Purchases	0	400,000
Settlements	0	0
Sales	0	(700,000)
Transfers out of Level 3		(3,800,000)
Balance at End of Period	30,300,000	33,200,000
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	45,600,000	44,000,000
Included in Condensed Consolidated Statements of Operations	(400,000)	(1,000,000)
Included in Other Comprehensive Income	(500,000)	300,000
Purchases	0	100,000
Settlements	0	0
Sales	(2,500,000)	0
Transfers out of Level 3		0
Balance at End of Period	$ 42,200,000	$ 43,400,000