Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 519.4		$ 551.3	$ 581.7	$ 556.6	$ 581.7
Included in Condensed Consolidated Statement of Operations	1.0		5.1		3.2	0.6
Included in Other Comprehensive Income (Loss)	0.2		(1.3)		(1.9)	(3.8)
Purchases	56.6		60.9		131.3	101.0
Settlements	(68.3)		(37.1)		(100.4)	(59.9)
Sales	(27.5)		(18.3)		(40.9)	(48.2)
Transfers into Level 3	0.0		0.0	0.0	0.3	0.0
Transfers out of Level 3	0.0		14.0	24.8	(66.8)	24.8
Balance at End of Period	481.4	$ 556.6	546.6	551.3	481.4	546.6
Fair Value, Level 2 to level 3 Transfers, Amount					(0.3)
Fair Value, Level 3 to level 1 Transfers, Amount		(3.5)		(3.5)
Fair Value, Level 3 to level 2 Transfers, Amount		(10.5)		(21.3)	(5.0)	21.3
Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	391.3		391.2	403.2	401.5	403.2
Included in Condensed Consolidated Statement of Operations	(0.4)		(0.6)		1.4	(3.9)
Included in Other Comprehensive Income (Loss)	1.7		1.4		0.1	2.8
Purchases	67.9		50.2		93.7	80.0
Settlements	(47.5)		(29.3)		(65.6)	(44.3)
Sales	(27.5)		(6.3)		(40.9)	(28.3)
Transfers into Level 3	0.0		0.0		0.3	0.0
Transfers out of Level 3	0.0		10.5		5.0	13.4
Balance at End of Period	385.5	401.5	396.1	391.2	385.5	396.1
States and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period			2.6	3.8		3.8
Included in Condensed Consolidated Statement of Operations			0.0			(1.2)
Included in Other Comprehensive Income (Loss)			0.0			0.0
Purchases			0.0			0.0
Settlements			(2.6)			(2.6)
Sales			0.0			0.0
Transfers into Level 3			0.0			0.0
Transfers out of Level 3			0.0			0.0
Balance at End of Period			0.0	2.6		0.0
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period			0.2	0.6	0.1	0.6
Included in Condensed Consolidated Statement of Operations			0.1		(0.1)	0.1
Included in Other Comprehensive Income (Loss)			(0.2)		0.0	(0.1)
Purchases			0.0		0.0	0.0
Settlements			0.0		0.0	0.5
Sales			0.0		0.0	0.0
Transfers into Level 3			0.0		0.0	0.0
Transfers out of Level 3			0.0		0.0	0.0
Balance at End of Period	0.0	0.1	0.1	0.2	0.0	0.1
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	128.1		95.8	103.5	93.2	103.5
Included in Condensed Consolidated Statement of Operations	1.4		0.7		1.9	0.9
Included in Other Comprehensive Income (Loss)	(1.5)		(0.2)		(2.0)	1.7
Purchases	(11.3)		7.2		37.6	16.4
Settlements	(20.8)		(5.2)		(34.8)	(12.5)
Sales	0.0		2.9		0.0	6.7
Transfers into Level 3	0.0		0.0		0.0	0.0
Transfers out of Level 3	0.0		0.0		0.0	(7.9)
Balance at End of Period	$ 95.9	$ 93.2	$ 95.4	$ 95.8	95.9	$ 95.4
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Transfers out of Level 3					$ (61.8)