Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Impairment Losses			$ (7.2)	$ (19.0)
Income Tax Benefit (Expense)	$ 27.1	$ (33.6)	28.7	(47.3)
Net Income	(62.6)	126.1	60.6	190.1
Stockholders' Equity Attributable to Parent	4,306.2		4,306.2			$ 4,563.4
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Income	12.5	2.1	19.3	4.6
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Interest and Other Expenses	(0.1)	0.0	(0.2)	(0.1)
Income Tax Benefit (Expense)	0.0	0.0	0.0	0.0
Net Income	(0.1)	0.0	(0.2)	(0.1)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	165.2		(116.3)
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	(12.6)		(20.3)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	152.6		(136.6)
Stockholders' Equity Attributable to Parent	594.0		594.0		$ 441.4	730.6
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Realized Gains on Sales of Investments	19.1	10.5	32.9	26.4
Impairment Losses	(3.2)	(7.0)	(7.2)	(19.0)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	15.9	3.5	25.7	7.4
Income Tax Benefit (Expense)	(3.3)	(0.8)	(5.4)	(1.6)
Net Income	12.6	2.7	20.3	5.8
Accumulated Other-than-Temporary Impairment Attributable to Parent [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	(0.2)		(1.9)
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	0.0		0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(0.2)		(1.9)
Stockholders' Equity Attributable to Parent	(4.0)		(4.0)		(3.8)	(2.1)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	0.0		0.0
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax			0.8
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.0		0.8
Stockholders' Equity Attributable to Parent	(44.9)		(44.9)		(44.9)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Interest and Other Expenses	(0.1)	(0.7)	(0.4)	(1.4)
Income Tax Benefit (Expense)	(0.1)	0.1	(0.4)	0.3
Net Income	0.0	$ (0.6)	(0.8)	$ (1.1)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	0.0		0.0
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	0.1		0.2
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.1		0.2
Stockholders' Equity Attributable to Parent	(2.1)		(2.1)		(2.2)
Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	165.0		(118.2)
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	(12.5)		(19.3)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	152.5		(137.5)
Stockholders' Equity Attributable to Parent	$ 543.0		$ 543.0		$ 390.5	$ 680.5