Correction of Misstatement in Prior Period Financial Statements Correction of Misstatement in Prior Period Financial Statements (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Premiums Earned, Net, Property and Casualty	$ 314.9		$ 318.1
Revenues	323.7		345.1
Policyholder Benefits and Claims Incurred, Net, Property, Casualty and Liability	(212.3)		(234.3)
Acquisition And Operating Expenses	110.8		111.5
Costs and Expenses	327.0		349.1
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(3.3)		(4.0)
Deferred Income Tax Expense (Benefit)	(0.2)		(0.6)
Income Tax Expense (Benefit)	(1.2)		(0.7)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	(2.1)		(3.3)
Increase (Decrease) in Deferred Policy Acquisition Costs	5.2		5.8
Increase (Decrease) in Accrued Investment Income Receivable	$ 6.3		$ (14.0)
Basic (usd per share)	$ (0.05)		$ (0.08)
Diluted (usd per share)	$ (0.05)		$ (0.08)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	$ (30.8)
Deferred Policy Acquisition Costs	105.1
Income Taxes Receivable, Current	5.7	$ 4.8		$ 4.8
Deferred Tax Assets, Net of Valuation Allowance	66.6
Assets	(2,957.3)	(3,008.4)		(3,008.4)
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability	1,247.6	1,255.6	$ 1,217.6	1,255.6	$ 1,181.6
Liability, Defined Benefit Plan	63.6	64.5		64.5
Other Liabilities	73.9	76.7		76.7
Liabilities	2,108.2	2,133.4		2,133.4
Retained Earnings (Accumulated Deficit)	710.7
Stockholders' Equity Attributable to Parent	849.1
Liabilities and Equity	2,957.3
Federal income taxes	(1.0)		(0.9)
Net Cash Provided by (Used in) Operating Activities	$ (10.7)		4.5
As Reported [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Premiums Earned, Net, Property and Casualty		318.7	317.3	1,275.1	1,291.9
Revenues		362.9	344.3	1,421.3	1,405.4
Policyholder Benefits and Claims Incurred, Net, Property, Casualty and Liability		(200.4)	(234.0)	(918.3)	(942.4)
Acquisition And Operating Expenses		121.0	112.2	455.8	430.4
Costs and Expenses		325.0	349.5	1,387.9	1,386.2
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest		37.9	(5.2)	33.4	19.2
Deferred Income Tax Expense (Benefit)		43.3	(1.0)	43.7	(0.1)
Income Tax Expense (Benefit)		43.7	(1.1)	44.1	(1.8)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		$ (5.8)	$ (4.1)	(10.7)	21.0
Increase (Decrease) in Deferred Policy Acquisition Costs				12.0	(0.7)
Increase (Decrease) in Accrued Investment Income Receivable				$ 6.0	$ 7.3
Basic (usd per share)		$ (0.14)	$ (0.10)	$ (0.25)	$ 0.50
Diluted (usd per share)		$ (0.14)	$ (0.10)	$ (0.25)	$ 0.50
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest		$ (17.0)	$ 11.3	$ (6.5)	$ 15.9
Deferred Policy Acquisition Costs		117.8		117.8	129.8
Income Taxes Receivable, Current		4.8		4.8	6.7
Deferred Tax Assets, Net of Valuation Allowance		57.2		57.2	102.1
Assets		(3,014.3)		(3,014.3)	(2,959.4)
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability		1,255.6		1,255.6	1,181.6
Liability, Defined Benefit Plan		64.5		64.5	74.4
Other Liabilities		76.7		76.7	69.8
Liabilities		2,133.4		2,133.4	2,068.1
Retained Earnings (Accumulated Deficit)		666.2		666.2	693.8
Stockholders' Equity Attributable to Parent		880.9		880.9	891.3
Liabilities and Equity		3,014.3		3,014.3	2,959.4
Federal income taxes				46.3	4.5
Net Cash Provided by (Used in) Operating Activities				67.9	113.5
Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Premiums Earned, Net, Property and Casualty		0.2	0.8	1.0	(0.8)
Revenues		0.2	0.8	1.0	(0.8)
Policyholder Benefits and Claims Incurred, Net, Property, Casualty and Liability		0.3	(0.3)	0.0	0.1
Acquisition And Operating Expenses		0.3	(0.7)	(0.6)	(0.3)
Costs and Expenses		0.0	(0.4)	(0.6)	(0.4)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest		0.2	1.2	1.6	(0.4)
Deferred Income Tax Expense (Benefit)		1.1	0.4	1.6	1.4
Income Tax Expense (Benefit)		1.1	0.4	1.6	1.4
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		$ (0.9)	$ 0.8	0.0	(1.8)
Increase (Decrease) in Deferred Policy Acquisition Costs				(0.5)	(0.7)
Increase (Decrease) in Accrued Investment Income Receivable				$ 0.2	$ 0.7
Basic (usd per share)		$ (0.02)	$ 0.02	$ 0	$ (0.04)
Diluted (usd per share)		$ (0.02)	$ 0.02	$ 0	$ (0.04)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest		$ (0.9)	$ 0.8	$ 0.0	$ (1.8)
Deferred Policy Acquisition Costs		(7.5)		(7.5)	(8.0)
Income Taxes Receivable, Current		0.0		0.0	0.0
Deferred Tax Assets, Net of Valuation Allowance		1.6		1.6	3.2
Assets		5.9		5.9	4.8
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability		0.0		0.0	0.0
Liability, Defined Benefit Plan		0.0		0.0	0.3
Other Liabilities		0.0		0.0	0.8
Liabilities		0.0		0.0	1.1
Retained Earnings (Accumulated Deficit)		(5.9)		(5.9)	(5.9)
Stockholders' Equity Attributable to Parent		(5.9)		(5.9)	(5.9)
Liabilities and Equity		(5.9)		(5.9)	(4.8)
Federal income taxes				0.3	1.8
Net Cash Provided by (Used in) Operating Activities				0.0	0.0
As Revised [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Premiums Earned, Net, Property and Casualty		318.9	318.1	1,276.1	1,291.1
Revenues		363.1	345.1	1,422.3	1,404.6
Policyholder Benefits and Claims Incurred, Net, Property, Casualty and Liability		(200.1)	(234.3)	(918.3)	(942.3)
Acquisition And Operating Expenses		121.3	111.5	455.2	430.1
Costs and Expenses		325.0	349.1	1,387.3	1,385.8
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest		38.1	(4.0)	35.0	18.8
Deferred Income Tax Expense (Benefit)		44.4	(0.6)	45.3	1.3
Income Tax Expense (Benefit)		44.8	(0.7)	45.7	(0.4)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		$ (6.7)	$ (3.3)	(10.7)	19.2
Increase (Decrease) in Deferred Policy Acquisition Costs				11.5	(1.4)
Increase (Decrease) in Accrued Investment Income Receivable				$ 6.2	$ 8.0
Basic (usd per share)		$ (0.16)	$ (0.08)	$ (0.25)	$ 0.46
Diluted (usd per share)		$ (0.16)	$ (0.08)	$ (0.25)	$ 0.46
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest		$ (17.9)	$ 12.1	$ (6.5)	$ 14.1
Deferred Policy Acquisition Costs		110.3		110.3	121.8
Income Taxes Receivable, Current		4.8		4.8	6.7
Deferred Tax Assets, Net of Valuation Allowance		58.8		58.8	105.3
Assets					(2,954.6)
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability		1,255.6		1,255.6	1,181.6
Liability, Defined Benefit Plan		64.5		64.5	74.7
Other Liabilities		76.7		76.7	70.6
Liabilities		2,133.4		2,133.4	2,069.2
Retained Earnings (Accumulated Deficit)		660.3		660.3	687.9
Stockholders' Equity Attributable to Parent		875.0		875.0	885.4
Liabilities and Equity		$ 3,008.4		3,008.4	2,954.6
Federal income taxes				46.6	6.3
Net Cash Provided by (Used in) Operating Activities				$ 67.9	$ 113.5