Income Taxes - Comprehensive Income Tax Benefit and Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Loss Carryforwards [Line Items]
Income Tax Expense	$ (23.1)	$ (29.8)	$ (33.6)	$ (13.7)	$ (32.2)	$ (32.5)	$ (26.4)	$ (39.4)	$ (100.2)	$ (130.5)	$ (10.7)
Unrealized Depreciation (Appreciation) on Securities									(78.3)	(85.2)	49.6
Foreign Currency Translation Adjustments on Investments									0.0	0.0	(0.1)
Tax Effects from Postretirement Benefit Plans									(15.3)	1.7	1.5
Changes in Gain (Loss) on Cash Flow Hedges									0.0	(0.1)	(0.3)
Comprehensive Income Tax (Expense) Benefit									(193.8)	(214.1)	39.4
Continuing Operations
Operating Loss Carryforwards [Line Items]
Income Tax Expense									(100.2)	(130.5)	(10.7)
Discontinued Operations
Operating Loss Carryforwards [Line Items]
Income Tax Expense									$ 0.0	$ 0.0	$ (0.6)