Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 950,700,000	$ 519,400,000	$ 897,400,000	$ 556,600,000
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	(4,800,000)	1,000,000.0	(7,600,000)	3,200,000
Included in Other Comprehensive Income (Loss)	6,800,000	200,000	18,700,000	(1,900,000)
Purchases	80,400,000	56,600,000	185,600,000	131,300,000
Settlements	(16,800,000)	(68,300,000)	(28,400,000)	(100,400,000)
Sales	(52,700,000)	(27,500,000)	(118,600,000)	(40,900,000)
Transfers into Level 3	3,000,000.0	0	19,500,000	300,000
Transfers out of Level 3	(1,600,000)	0	(1,600,000)	(66,800,000)
Balance at End of Period	965,000,000.0	481,400,000	965,000,000.0	481,400,000
Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	411,900,000	391,300,000	382,600,000	401,500,000
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	(5,200,000)	(400,000)	(7,700,000)	1,400,000
Included in Other Comprehensive Income (Loss)	4,800,000	1,700,000	10,200,000	100,000
Purchases	60,100,000	67,900,000	161,400,000	93,700,000
Settlements	(4,600,000)	(47,500,000)	(16,100,000)	(65,600,000)
Sales	(49,800,000)	(27,500,000)	(115,700,000)	(40,900,000)
Transfers into Level 3	0	0	2,500,000	300,000
Transfers out of Level 3	(1,600,000)	0	(1,600,000)	(5,000,000.0)
Balance at End of Period	415,600,000	385,500,000	415,600,000	385,500,000
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period				100,000
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations				(100,000)
Included in Other Comprehensive Income (Loss)				0
Purchases				0
Settlements				0
Sales				0
Transfers into Level 3				0
Transfers out of Level 3				0
Balance at End of Period		0		0
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	528,500,000	128,100,000	504,900,000	93,200,000
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	400,000	1,400,000	100,000	1,900,000
Included in Other Comprehensive Income (Loss)	1,700,000	(1,500,000)	7,700,000	(2,000,000.0)
Purchases	20,300,000	(11,300,000)	24,200,000	37,600,000
Settlements	(12,000,000.0)	(20,800,000)	(12,000,000.0)	(34,800,000)
Sales	(2,900,000)	0	(2,900,000)	0
Transfers into Level 3	3,000,000.0	0	17,000,000.0	0
Transfers out of Level 3	0	0	0	0
Balance at End of Period	539,000,000.0	95,900,000	539,000,000.0	95,900,000
Other Mortgage- and Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	10,300,000		9,900,000
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	0		0
Included in Other Comprehensive Income (Loss)	300,000		800,000
Purchases	0		0
Settlements	(200,000)		(300,000)
Sales	0		0
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Balance at End of Period	$ 10,400,000		$ 10,400,000
Preferred and Common Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period				27,400,000
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations				0
Included in Other Comprehensive Income (Loss)				0
Purchases				0
Settlements				0
Sales				0
Transfers into Level 3				0
Transfers out of Level 3				(27,400,000)
Balance at End of Period		0		0
Other Equity Interests
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period				34,400,000
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations				0
Included in Other Comprehensive Income (Loss)				0
Purchases				0
Settlements				0
Sales				0
Transfers into Level 3				0
Transfers out of Level 3				(34,400,000)
Balance at End of Period		$ 0		$ 0