Federal Income Taxes - Tax Effects of Temporary Differences of Deferred Tax Assets and Deferred Tax Liabilities (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Unearned premiums not currently deductible	$ 24,300,000	$ 25,500,000
Losses and loss expenses payable discounting	19,900,000	22,000,000.0
Postretirement and pension benefits	21,400,000
Realized loss on other-than-temporary impairment	1,900,000
Other liabilities	14,800,000
Net operating loss carryforward	15,100,000
Tax credit carryforward	2,700,000
Other	1,700,000
Total deferred tax assets	107,900,000
Deferred tax liabilities:
Deferral of policy acquisition costs	21,400,000	23,100,000
Net unrealized holding gains on investments	0	20,300,000
Losses and loss expenses payable discounting (transition rule)	8,700,000	$ 10,700,000
Total deferred tax liabilities	30,100,000
Net deferred federal income taxes	$ 77,800,000