Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets
Unearned premium reserve reduction	$ 11,472	$ 18,253
Compensation accruals	8,359	15,893
Impaired securities	2,240	8,214
Other comprehensive income - net funded status of pension and other postretirement benefit obligations	3,526	6,387
Discounting of unpaid claims and claim expense tax reserves	3,889	2,463
Postretirement benefits other than pensions	321	578
Total gross deferred tax assets	29,807	51,788
Deferred tax liabilities
Other comprehensive income - net unrealized gains on fixed maturity and equity securities	95,583	112,311
Deferred policy acquisition costs	52,438	91,028
Life insurance future policy benefit reserve	102	33,145
Life insurance future policy benefit reserve (transitional rule)	23,869	0
Discounting of unpaid claims and claim expense tax reserves (transitional rule)	2,513	0
Investment related adjustments	8,127	15,762
Intangible assets	2,557	4,262
Other, net	2,393	979
Total gross deferred tax liabilities	187,582	257,487
Net deferred tax liability	$ 157,775	$ 205,699