Schedule 2 - Parent Company Financial Statements - Statements of Comprehensive Income (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
Condensed Statement of Income Captions [Line Items]
Net Income	$ 124.7	$ 129.0	$ 122.1	$ 155.3	$ 6.5	$ 92.2	$ 37.6	$ 53.8	$ 531.1	$ 190.1	$ 120.9
Unrealized Holding Gains (Losses) Arising During the Year:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment									433.4	(214.2)	119.1
Reclassification Adjustment for Amounts Included in Net Income:
Reclassification Adjustment for Amounts Included in Net Income									(28.1)	(21.9)	(35.3)
Unrealized Holding Gains (Losses)									405.3	(236.1)	83.8
Unrecognized Postretirement Benefit Costs Arising During the Year									(4.8)	(8.0)	3.9
Reclassification Adjustments for Amounts Included in Net Income:
Total Reclassification Adjustments for Amounts Included in Net Income									(3.0)	1.1	(0.6)
Net Unrecognized Postretirement Benefit Costs									(7.8)	(6.9)	3.3
Foreign Currency Translation Adjustments									0.0	0.3	1.7
Gains (Losses) on Cash Flow Hedge									0.4	1.2	(6.7)
Other Comprehensive Income (Loss) Before Income Taxes									397.9	(241.5)	82.1
Unrealized Holding Gains and Losses Arising During the Year:
Unrealized Holding Gains (Losses) Arising During the Year Before Reclassification Adjustment									(91.0)	45.0	(38.2)
Reclassification Adjustment for Amounts Included in Net Income:
Reclassification Adjustment for Amounts Included in Net Income									(5.8)	(4.6)	(12.3)
Unrealized Holding Gains and Losses									(85.2)	49.6	(25.9)
Unrecognized Postretirement Benefit Costs Arising During the Year									1.0	1.7	(0.8)
Reclassification Adjustments for Amounts Included in Net Income:
Total Reclassification Adjustments for Amounts Included in Net Income									0.7	(0.2)	0.2
Net Unrecognized Postretirement Benefit Costs									1.7	1.5	(0.6)
Foreign Currency Translation Adjustments on Investments									0.0	(0.1)	(0.6)
Gains (Losses) on Cash Flow Hedge									(0.1)	(0.3)	2.4
Other Comprehensive Income Tax Benefit (Expense)									(83.6)	50.7	(24.7)
Other Comprehensive Income (Loss)									314.3	(190.8)	57.4
Total Comprehensive Income (Loss)									845.4	(0.7)	178.3
Subsidiaries
Unrealized Holding Gains (Losses) Arising During the Year:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment									433.2	(214.1)	119.5
Reclassification Adjustment for Amounts Included in Net Income:
Reclassification Adjustment for Amounts Included in Net Income									(27.9)	(21.9)	(35.3)
Unrecognized Postretirement Benefit Costs Arising During the Year									(0.6)	0.0	0.0
Reclassification Adjustments for Amounts Included in Net Income:
Foreign Currency Translation Adjustments									0.0	0.3	1.7
Unrealized Holding Gains and Losses Arising During the Year:
Unrealized Holding Gains (Losses) Arising During the Year Before Reclassification Adjustment									(91.0)	45.0	(38.3)
Reclassification Adjustment for Amounts Included in Net Income:
Reclassification Adjustment for Amounts Included in Net Income									5.8	4.6	12.3
Reclassification Adjustments for Amounts Included in Net Income:
Foreign Currency Translation Adjustments on Investments									0.0	(0.1)	(0.6)
Parent Company
Condensed Statement of Income Captions [Line Items]
Net Income									531.1	190.1	120.9
Unrealized Holding Gains (Losses) Arising During the Year:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment									0.2	(0.1)	(0.4)
Reclassification Adjustment for Amounts Included in Net Income:
Reclassification Adjustment for Amounts Included in Net Income									(0.2)	0.0	0.0
Unrealized Holding Gains (Losses)									405.3	(236.1)	83.8
Unrecognized Postretirement Benefit Costs Arising During the Year									(4.2)	(8.0)	3.9
Reclassification Adjustments for Amounts Included in Net Income:
Amortization of Unrecognized Postretirement Benefits (Costs)									(3.0)	1.1	(0.6)
Total Reclassification Adjustments for Amounts Included in Net Income									(3.0)	1.1	(0.6)
Net Unrecognized Postretirement Benefit Costs									(7.8)	(6.9)	3.3
Gains (Losses) on Cash Flow Hedge									0.4	1.2	(6.7)
Other Comprehensive Income (Loss) Before Income Taxes									397.9	(241.5)	82.1
Unrealized Holding Gains and Losses Arising During the Year:
Unrealized Holding Gains (Losses) Arising During the Year Before Reclassification Adjustment									0.0	0.0	0.1
Reclassification Adjustment for Amounts Included in Net Income:
Unrealized Holding Gains and Losses									(85.2)	49.6	(25.9)
Unrecognized Postretirement Benefit Costs Arising During the Year									1.0	1.7	(0.8)
Reclassification Adjustments for Amounts Included in Net Income:
Amortization of Unrecognized Postretirement Benefit Costs									0.7	(0.2)	0.2
Total Reclassification Adjustments for Amounts Included in Net Income									0.7	(0.2)	0.2
Net Unrecognized Postretirement Benefit Costs									1.7	1.5	(0.6)
Gains (Losses) on Cash Flow Hedge									(0.1)	(0.3)	2.4
Other Comprehensive Income Tax Benefit (Expense)									(83.6)	50.7	(24.7)
Other Comprehensive Income (Loss)									314.3	(190.8)	57.4
Total Comprehensive Income (Loss)									$ 845.4	$ (0.7)	$ 178.3