Schedule 2 - Parent Company Financial Statements - Statements of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net Income	$ 36.9	$ 47.7	$ 36.6	$ (0.3)	$ 31.2	$ (16.3)	$ 4.0	$ (2.1)	$ 120.9	$ 16.8	$ 85.7
Unrealized Holding Gains (Losses) Arising During the Year:
Securities Held by Parent									119.1	(2.4)	(151.9)
Reclassification Adjustment for Amounts Included in Net Income:
Securities Held by Parent									(35.3)	0.2	(25.4)
Unrealized Holding Gains (Losses)									83.8	(2.2)	(177.3)
Unrecognized Postretirement Benefit Costs Arising During the Year									3.9	14.2	3.0
Other Comprehensive Income (Loss), Finalization of Pension and Other Postretirement Benefit Plan Valuation, before Tax									0.0	1.0	0.0
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, before Tax									(0.6)	6.3	23.1
Net Unrecognized Postretirement Benefit Costs									3.3	20.5	26.1
Foreign Currency Translation Adjustments									1.7	(0.3)	(1.4)
Reclassification Adjustment for Amounts Included in Net Income									0.0	0.0	0.0
Gain on Cash Flow Hedge									(6.7)	1.6	0.0
Other Comprehensive Income (Loss) Before Income Taxes									82.1	19.6	(152.6)
Income Tax Benefit (Expense):
Securities Held by Parent									(38.2)	1.0	53.9
Securities Held by Parent									12.3	(0.1)	8.9
Unrealized Holding Gains and Losses									(25.9)	0.9	62.8
Unrecognized Postretirement Benefit Costs Arising During the Year									(0.8)	(5.0)	(1.1)
Curtailment Cost Recognized									0.0	0.4	0.0
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Tax									0.2	(2.3)	(8.0)
Net Unrecognized Postretirement Benefit Costs									(0.6)	(7.3)	(9.1)
Foreign Currency Translation Adjustments									(0.6)	0.1	0.5
Reclassification Adjustment for Amounts Included in Net Income									0.0	0.0	0.0
Gain on Cash Flow Hedge									2.4	(0.6)	0.0
Other Comprehensive Income Tax Benefit (Expense)									(24.7)	(6.9)	54.2
Other Comprehensive Income (Loss), Net of Tax									57.4	12.7	(98.4)
Total Comprehensive Income (Loss)									178.3	29.5	(12.7)
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Income									120.9	16.8	85.7
Unrealized Holding Gains (Losses) Arising During the Year:
Securities Held by Subsidiaries									119.5	(2.6)	(150.4)
Securities Held by Parent									(0.4)	0.2	(1.5)
Reclassification Adjustment for Amounts Included in Net Income:
Securities Held by Subsidiaries									(35.3)	0.2	(26.9)
Securities Held by Parent									0.0	0.0	1.5
Unrealized Holding Gains (Losses)									83.8	(2.2)	(177.3)
Unrecognized Postretirement Benefit Costs Arising During the Year									3.9	14.2	3.0
Other Comprehensive Income (Loss), Finalization of Pension and Other Postretirement Benefit Plan Valuation, before Tax									0.0	1.0	0.0
Amortization of Unrecognized Postretirement Benefit Costs									(0.6)	5.3	23.1
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, before Tax									(0.6)	6.3	23.1
Net Unrecognized Postretirement Benefit Costs									3.3	20.5	26.1
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Held by Subsidiaries, before Tax									1.7	(0.3)	(1.4)
Gain on Cash Flow Hedge									(6.7)	1.6	0.0
Other Comprehensive Income (Loss) Before Income Taxes									82.1	19.6	(152.6)
Income Tax Benefit (Expense):
Securities Held by Subsidiaries									(38.3)	1.1	53.4
Securities Held by Parent									0.1	(0.1)	0.5
Securities Held by Subsidiaries									12.3	(0.1)	9.4
Securities Held by Parent									0.0	0.0	(0.5)
Unrealized Holding Gains and Losses									(25.9)	0.9	62.8
Unrecognized Postretirement Benefit Costs Arising During the Year									(0.8)	(5.0)	(1.1)
Curtailment Cost Recognized									0.0	(0.4)	0.0
Amortization of Unrecognized Postretirement Benefit Costs									0.2	(1.9)	(8.0)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Tax									0.2	(2.3)	(8.0)
Net Unrecognized Postretirement Benefit Costs									(0.6)	(7.3)	(9.1)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Held by Subsidiaries, Tax									(0.6)	0.1	0.5
Gain on Cash Flow Hedge									2.4	(0.6)	0.0
Other Comprehensive Income Tax Benefit (Expense)									(24.7)	(6.9)	54.2
Other Comprehensive Income (Loss), Net of Tax									57.4	12.7	(98.4)
Total Comprehensive Income (Loss)									$ 178.3	$ 29.5	$ (12.7)