Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		$ 571,500,000	$ 606,800,000	$ 518,400,000
Included in Condensed Consolidated Statement of Operations		(600,000)	(15,800,000)	(6,100,000)
Included in Other Comprehensive Income (Loss)		(6,000,000)	5,100,000	(3,500,000)
Purchases		66,600,000	183,800,000	223,100,000
Settlements		(29,100,000)	(40,800,000)	(44,100,000)
Sales		(7,400,000)	(89,000,000)	(72,900,000)
Transfers into Level 3	$ 4,100,000	0	4,100,000	800,000
Transfers out of Level 3	(25,900,000)	0	(25,900,000)	(20,700,000)
Balance at End of Period	628,300,000	595,000,000	628,300,000	595,000,000
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	459,400,000	406,600,000	436,300,000	360,600,000
Included in Condensed Consolidated Statement of Operations	(7,000,000)	(400,000)	(15,800,000)	(3,500,000)
Included in Other Comprehensive Income (Loss)	(900,000)	(2,000,000)	2,400,000	(3,400,000)
Purchases	60,100,000	59,400,000	148,300,000	184,700,000
Settlements	(20,600,000)	(28,300,000)	(34,900,000)	(35,200,000)
Sales	(34,000,000)	(6,800,000)	(79,300,000)	(62,400,000)
Transfers into Level 3	0		0	0
Transfers out of Level 3	(15,000,000)		(15,000,000)	(12,300,000)
Balance at End of Period	442,000,000	428,500,000	442,000,000	428,500,000
US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	0		0
Included in Condensed Consolidated Statement of Operations	0		0
Included in Other Comprehensive Income (Loss)	0		0
Purchases	0		0
Settlements	0		0
Sales	0		0
Transfers into Level 3	4,100,000		4,100,000
Transfers out of Level 3	0		0
Balance at End of Period	4,100,000		4,100,000
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	2,900,000	5,400,000	3,800,000	6,700,000
Included in Condensed Consolidated Statement of Operations	0	(800,000)	0	(1,300,000)
Included in Other Comprehensive Income (Loss)	400,000	0	0	(600,000)
Purchases	0	0	0	0
Settlements	(2,400,000)	(100,000)	(2,900,000)	(300,000)
Sales	0	0	0	0
Transfers into Level 3	0		0	0
Transfers out of Level 3	0		0	0
Balance at End of Period	900,000	4,500,000	900,000	4,500,000
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	102,200,000	70,400,000	87,300,000	64,400,000
Included in Condensed Consolidated Statement of Operations	100,000	100,000	(200,000)	500,000
Included in Other Comprehensive Income (Loss)	5,200,000	(1,500,000)	3,000,000	(300,000)
Purchases	13,900,000	5,300,000	33,200,000	24,700,000
Settlements	0	0	0	(7,500,000)
Sales	0	0	(1,900,000)	(7,500,000)
Transfers into Level 3	0		0	0
Transfers out of Level 3	(10,100,000)		(10,100,000)	0
Balance at End of Period	111,300,000	74,300,000	111,300,000	74,300,000
Other Mortgage- and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	3,700,000	4,000,000	3,800,000	3,900,000
Included in Condensed Consolidated Statement of Operations	400,000	0	400,000	0
Included in Other Comprehensive Income (Loss)	(400,000)	(100,000)	(400,000)	0
Purchases	0	0	0	1,300,000
Settlements	(2,900,000)	0	(3,000,000)	(100,000)
Sales	0	0	0	(1,200,000)
Transfers into Level 3	0		0	0
Transfers out of Level 3	(800,000)		(800,000)	0
Balance at End of Period	0	3,900,000	0	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	31,400,000	37,700,000	30,000,000	38,800,000
Included in Condensed Consolidated Statement of Operations	1,400,000	0	500,000	(1,200,000)
Included in Other Comprehensive Income (Loss)	1,000,000	(600,000)	3,500,000	1,600,000
Purchases	100,000	200,000	200,000	7,200,000
Settlements	0	(200,000)	0	(500,000)
Sales	(2,900,000)	(600,000)	(3,200,000)	(1,800,000)
Transfers into Level 3	0		0	800,000
Transfers out of Level 3	0		0	(8,400,000)
Balance at End of Period	31,000,000	36,500,000	31,000,000	36,500,000
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	47,200,000	47,400,000	45,600,000	44,000,000
Included in Condensed Consolidated Statement of Operations	0	500,000	(700,000)	(600,000)
Included in Other Comprehensive Income (Loss)	(6,200,000)	(1,800,000)	(3,400,000)	(800,000)
Purchases	100,000	1,700,000	2,100,000	5,200,000
Settlements	0	(500,000)	0	(500,000)
Sales	(2,100,000)	0	(4,600,000)	0
Transfers into Level 3	0		0	0
Transfers out of Level 3	0		0	0
Balance at End of Period	39,000,000	$ 47,300,000	39,000,000	$ 47,300,000
Fair Value Option Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	646,800,000
Included in Condensed Consolidated Statement of Operations	(5,100,000)
Included in Other Comprehensive Income (Loss)	(900,000)
Purchases	74,200,000
Settlements	(25,900,000)
Sales	(39,000,000)
Transfers into Level 3	4,100,000
Transfers out of Level 3	(25,900,000)
Balance at End of Period	$ 628,300,000		$ 628,300,000