Correction of Misstatement in Prior Period Financial Statements (Details) - USD ($) $ / shares in Units, $ 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	Dec. 31, 2015
Error Corrections and Prior Period Adjustments Restatement
Premiums Earned, Net, Property and Casualty									$ 318.3	$ 459.0	$ 471.2
Revenues	$ 264.4	$ 346.0	$ 341.7	$ 323.7	$ 363.1	$ 358.4	$ 355.7	$ 345.1	1,275.8	1,422.3	1,404.6
Policyholder Benefits and Claims Incurred, Net, Property, Casualty and Liability									(796.4)	(918.3)	(942.3)
Acquisition And Operating Expenses									449.8	455.2	430.1
Income (loss) before federal income tax									12.9	35.0	18.8
Deferred Income Tax Expense (Benefit)									1.2	52.4	1.3
Total federal income tax expense (benefit)									0.1	52.8	(0.4)
Net (loss) income	$ (24.5)	$ 33.4	$ 6.0	$ (2.1)	$ (13.8)	$ (9.5)	$ 8.7	$ (3.3)	12.8	(17.8)	19.2
Increase (Decrease) in Deferred Policy Acquisition Costs									8.4	11.5	(1.4)
Other liabilities and due from affiliates									$ 47.6	$ 11.3	$ 33.7
Basic (usd per share)	$ (0.57)	$ 0.78	$ 0.14	$ (0.05)	$ (0.33)	$ (0.23)	$ 0.21	$ (0.08)	$ 0.30	$ (0.42)	$ 0.46
Diluted (usd per share)	$ (0.57)	$ 0.76	$ 0.14	$ (0.05)	$ (0.33)	$ (0.23)	$ 0.21	$ (0.08)	$ 0.29	$ (0.42)	$ 0.46
Net actuarial (loss) gain arising during period									$ (4.3)	$ (1.5)	$ (4.6)
Negative prior service cost									(6.3)	(6.3)	(6.2)
Net actuarial gain									13.2	12.5	14.9
Income tax expense									(0.6)	(1.7)	(1.5)
Total net unrecognized benefit plan obligations									2.0	3.0	2.6
Net current period other comprehensive income									(36.3)	6.2	(3.1)
Comprehensive Income (loss)									(23.5)	(11.6)	16.1
Deferred acquisition costs	$ 101.9				$ 110.3				101.9	110.3	121.8	$ 120.4
Current federal income taxes	5.9				4.8				5.9	4.8
Federal income tax, net	77.8				69.5				77.8	69.5
Assets	(2,895.9)				(3,019.1)				(2,895.9)	(3,019.1)
Losses and loss expenses payable (affiliated net assumed $711.4 and $630.9, respectively)	1,146.8				1,255.6				1,146.8	1,255.6	1,181.6	1,053.0
Liability, Defined Benefit Plan	83.0				99.3				83.0	99.3
Due to affiliate	22.4				18.6				22.4	18.6
Other liabilities	119.0				76.7				119.0	76.7
Liabilities	2,077.4				2,184.1				2,077.4	2,184.1
Accumulated other comprehensive income (affiliated net ceded $50.7 and $53.7, respectively)	(96.4)				3.8				(96.4)	3.8	(2.4)	$ 0.7
Retained earnings	712.7				653.2				712.7	653.2
Total stockholders’ equity	818.5				835.0				818.5	835.0	850.5
Total liabilities and stockholders' equity	$ 2,895.9				3,019.1				2,895.9	3,019.1
Postretirement and pension benefits									(16.9)	(14.8)	(48.0)
Federal income taxes									(0.8)	54.4	(0.2)
Net cash provided by operating activities									$ (13.4)	67.9	113.5
Previously Reported [Member]
Error Corrections and Prior Period Adjustments Restatement
Premiums Earned, Net, Property and Casualty										1,275.1	1,291.9
Revenues										1,421.3	1,405.4
Policyholder Benefits and Claims Incurred, Net, Property, Casualty and Liability										(918.3)	(942.4)
Acquisition And Operating Expenses										455.8	430.4
Costs and Expenses										1,387.9	1,386.2
Income (loss) before federal income tax										33.4	19.2
Deferred Income Tax Expense (Benefit)										43.7	(0.1)
Total federal income tax expense (benefit)										44.1	(1.8)
Net (loss) income										(10.7)	21.0
Increase (Decrease) in Deferred Policy Acquisition Costs										12.0	(0.7)
Other liabilities and due from affiliates										$ 6.0	$ 7.3
Basic (usd per share)										$ (0.25)	$ 0.50
Diluted (usd per share)										$ (0.25)	$ 0.50
Net actuarial (loss) gain arising during period										$ (1.0)	$ (3.0)
Negative prior service cost										5.5	5.5
Net actuarial gain										8.0	9.4
Income tax expense										(0.5)	(0.3)
Total net unrecognized benefit plan obligations										(1.0)	(0.6)
Net current period other comprehensive income										4.2	(5.1)
Comprehensive Income (loss)										(6.5)	15.9
Deferred acquisition costs					117.8					117.8	129.8
Current federal income taxes					4.8					4.8	6.7
Federal income tax, net					57.2					57.2	102.1
Assets					(3,014.3)					(3,014.3)	(2,959.4)
Losses and loss expenses payable (affiliated net assumed $711.4 and $630.9, respectively)					1,255.6					1,255.6	1,181.6
Liability, Defined Benefit Plan					64.5					64.5	74.4
Due to affiliate					2.7					2.7	2.4
Other liabilities					76.7					76.7	69.8
Liabilities					2,133.4					2,133.4	2,068.1
Accumulated other comprehensive income (affiliated net ceded $50.7 and $53.7, respectively)					36.7					36.7	32.5
Retained earnings					666.2					666.2	693.8
Total stockholders’ equity					880.9					880.9	891.3
Total liabilities and stockholders' equity					3,014.3					3,014.3	2,959.4
Postretirement and pension benefits										(9.0)	(28.8)
Federal income taxes										46.3	4.5
Net cash provided by operating activities										67.9	113.5
Scenario, Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement
Premiums Earned, Net, Property and Casualty										1.0	(0.8)
Revenues										1.0	(0.8)
Policyholder Benefits and Claims Incurred, Net, Property, Casualty and Liability										0.0	0.1
Acquisition And Operating Expenses										(0.6)	(0.3)
Costs and Expenses										(0.6)	(0.4)
Income (loss) before federal income tax										1.6	(0.4)
Deferred Income Tax Expense (Benefit)										8.7	1.4
Total federal income tax expense (benefit)										8.7	1.4
Net (loss) income										(7.1)	(1.8)
Increase (Decrease) in Deferred Policy Acquisition Costs										(0.5)	(0.7)
Other liabilities and due from affiliates										$ 5.3	$ 26.4
Basic (usd per share)										$ (0.17)	$ (0.04)
Diluted (usd per share)										$ (0.17)	$ (0.04)
Net actuarial (loss) gain arising during period										$ (0.5)	$ (1.6)
Negative prior service cost										0.8	0.7
Net actuarial gain										4.5	5.5
Income tax expense										(1.2)	(1.2)
Total net unrecognized benefit plan obligations										(2.0)	(2.0)
Net current period other comprehensive income										2.0	2.0
Comprehensive Income (loss)										(5.1)	0.2
Deferred acquisition costs					(7.5)					(7.5)	(8.0)
Current federal income taxes					0.0					0.0	0.0
Federal income tax, net					12.3					12.3	22.0
Assets					(4.8)					(4.8)	(14.0)
Losses and loss expenses payable (affiliated net assumed $711.4 and $630.9, respectively)					0.0					0.0	0.0
Liability, Defined Benefit Plan					34.8					34.8	40.4
Due to affiliate					15.9					15.9	13.6
Other liabilities					0.0					0.0	0.8
Liabilities					50.7					50.7	54.8
Accumulated other comprehensive income (affiliated net ceded $50.7 and $53.7, respectively)					(32.9)					(32.9)	(34.9)
Retained earnings					(13.0)					(13.0)	(5.9)
Total stockholders’ equity					(45.9)					(45.9)	(40.8)
Total liabilities and stockholders' equity					4.8					4.8	14.0
Postretirement and pension benefits										(5.8)	(19.2)
Federal income taxes										8.1	(4.7)
Net cash provided by operating activities										0.0	0.0
Restatement Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement
Premiums Earned, Net, Property and Casualty										1,276.1	1,291.1
Revenues										1,422.3	1,404.6
Policyholder Benefits and Claims Incurred, Net, Property, Casualty and Liability										(918.3)	(942.3)
Acquisition And Operating Expenses										455.2	430.1
Costs and Expenses										1,387.3	1,385.8
Income (loss) before federal income tax										35.0	18.8
Deferred Income Tax Expense (Benefit)										52.4	1.3
Total federal income tax expense (benefit)										52.8	(0.4)
Net (loss) income										(17.8)	19.2
Increase (Decrease) in Deferred Policy Acquisition Costs										11.5	(1.4)
Other liabilities and due from affiliates										$ 11.3	$ 33.7
Basic (usd per share)										$ (0.42)	$ 0.46
Diluted (usd per share)										$ (0.42)	$ 0.46
Net actuarial (loss) gain arising during period										$ (1.5)	$ (4.6)
Negative prior service cost										6.3	6.2
Net actuarial gain										12.5	14.9
Income tax expense										(1.7)	(1.5)
Total net unrecognized benefit plan obligations										(3.0)	(2.6)
Net current period other comprehensive income										6.2	(3.1)
Comprehensive Income (loss)										(11.6)	16.1
Deferred acquisition costs					110.3					110.3	121.8
Current federal income taxes					4.8					4.8	6.7
Federal income tax, net					69.5					69.5	124.1
Assets					(3,019.1)					(3,019.1)	(2,973.4)
Losses and loss expenses payable (affiliated net assumed $711.4 and $630.9, respectively)					1,255.6					1,255.6	1,181.6
Liability, Defined Benefit Plan					99.3					99.3	114.8
Due to affiliate					18.6					18.6	16.0
Other liabilities					76.7					76.7	70.6
Liabilities					2,184.1					2,184.1	2,122.9
Accumulated other comprehensive income (affiliated net ceded $50.7 and $53.7, respectively)					3.8					3.8	(2.4)
Retained earnings					653.2					653.2	687.9
Total stockholders’ equity					835.0					835.0	850.5
Total liabilities and stockholders' equity					$ 3,019.1					3,019.1	2,973.4
Postretirement and pension benefits										(14.8)	(48.0)
Federal income taxes										54.4	(0.2)
Net cash provided by operating activities										$ 67.9	$ 113.5