Schedule 2 - Parent Company Financial Statements - Statements of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net Income	$ 6.5	$ 92.2	$ 37.6	$ 53.8	$ 36.9	$ 47.7	$ 36.6	$ (0.3)	$ 190.1	$ 120.9	$ 16.8
Unrealized Holding Gains (Losses) Arising During the Year:
Securities Held by Parent									(214.2)	119.1	(2.4)
Reclassification Adjustment for Amounts Included in Net Income:
Securities Held by Parent									(21.9)	(35.3)	0.2
Unrealized Holding Gains (Losses)									(236.1)	83.8	(2.2)
Unrecognized Postretirement Benefit Costs Arising During the Year									(8.0)	3.9	14.2
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax									0.0	0.0	1.0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, before Tax									1.1	(0.6)	6.3
Net Unrecognized Postretirement Benefit Costs									(6.9)	3.3	20.5
Foreign Currency Translation Adjustments									0.0	1.7	(0.3)
Reclassification Adjustment for Amounts Included in Net Income									(0.3)	0.0	0.0
Gain on Cash Flow Hedge									1.2	(6.7)	1.6
Other Comprehensive Income (Loss) Before Income Taxes									(241.5)	82.1	19.6
Income Tax Benefit (Expense):
Securities Held by Parent									45.0	(38.2)	1.0
Securities Held by Parent									4.6	12.3	(0.1)
Unrealized Holding Gains and Losses									49.6	(25.9)	0.9
Unrecognized Postretirement Benefit Costs Arising During the Year									1.7	(0.8)	(5.0)
Curtailment Cost Recognized									0.0	0.0	0.4
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax									(0.2)	0.2	(2.3)
Net Unrecognized Postretirement Benefit Costs									1.5	(0.6)	(7.3)
Foreign Currency Translation Adjustments									0.0	(0.6)	0.1
Reclassification Adjustment for Amounts Included in Net Income									0.1	0.0	0.0
Gain on Cash Flow Hedge									(0.3)	2.4	(0.6)
Other Comprehensive Income Tax Benefit (Expense)									50.7	(24.7)	(6.9)
Other Comprehensive Income (Loss), Net of Tax									(190.8)	57.4	12.7
Total Comprehensive Income (Loss)									(0.7)	178.3	29.5
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Income									190.1	120.9	16.8
Unrealized Holding Gains (Losses) Arising During the Year:
Securities Held by Subsidiaries									(214.1)	119.5	(2.6)
Securities Held by Parent									(0.1)	(0.4)	0.2
Reclassification Adjustment for Amounts Included in Net Income:
Securities Held by Subsidiaries									(21.9)	(35.3)	0.2
Securities Held by Parent									0.0	0.0	0.0
Unrealized Holding Gains (Losses)									(236.1)	83.8	(2.2)
Unrecognized Postretirement Benefit Costs Arising During the Year									(8.0)	3.9	14.2
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax									0.0	0.0	1.0
Amortization of Unrecognized Postretirement Benefit Costs									1.1	(0.6)	5.3
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, before Tax									1.1	(0.6)	6.3
Net Unrecognized Postretirement Benefit Costs									(6.9)	3.3	20.5
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Held by Subsidiaries, before Tax									0.3	1.7	(0.3)
Gain on Cash Flow Hedge									1.2	(6.7)	1.6
Other Comprehensive Income (Loss) Before Income Taxes									(241.5)	82.1	19.6
Income Tax Benefit (Expense):
Securities Held by Subsidiaries									45.0	(38.3)	1.1
Securities Held by Parent									0.0	0.1	(0.1)
Securities Held by Subsidiaries									4.6	12.3	(0.1)
Unrealized Holding Gains and Losses									49.6	(25.9)	0.9
Unrecognized Postretirement Benefit Costs Arising During the Year									1.7	(0.8)	(5.0)
Curtailment Cost Recognized									0.0	0.0	(0.4)
Amortization of Unrecognized Postretirement Benefit Costs									(0.2)	0.2	(1.9)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax									(0.2)	0.2	(2.3)
Net Unrecognized Postretirement Benefit Costs									1.5	(0.6)	(7.3)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Held by Subsidiaries, Tax									(0.1)	(0.6)	0.1
Gain on Cash Flow Hedge									(0.3)	2.4	(0.6)
Other Comprehensive Income Tax Benefit (Expense)									50.7	(24.7)	(6.9)
Other Comprehensive Income (Loss), Net of Tax									(190.8)	57.4	12.7
Total Comprehensive Income (Loss)									$ (0.7)	$ 178.3	$ 29.5