Income Taxes - Comprehensive Income Tax Benefit and Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Income Tax Expense	$ (32.2)	$ (32.5)	$ (26.4)	$ (39.4)	$ (1.1)	$ 11.8	$ (8.0)	$ (13.4)	$ (130.5)	$ (10.7)	$ (41.2)
Unrealized Depreciation (Appreciation) on Securities									(85.2)	49.6	(25.9)
Foreign Currency Translation Adjustments on Investments									0.0	(0.1)	(0.6)
Tax Effects from Postretirement Benefit Plans									1.7	1.5	(0.6)
Gains (Losses) on Cash Flow Hedge									(0.1)	(0.3)	2.4
Comprehensive Income Tax (Expense) Benefit									(214.1)	39.4	(66.4)
Continuing Operations
Operating Loss Carryforwards [Line Items]
Income Tax Expense									(130.5)	(10.7)	(41.2)
Discontinued Operations
Operating Loss Carryforwards [Line Items]
Income Tax Expense									$ 0.0	$ (0.6)	$ (0.5)