Investments - Summary of Financial Information (Detail) - Donegal Financial Services Corporation [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Balance sheets:
Total assets	$ 543,086		$ 543,086		$ 567,935
Total liabilities	460,292		460,292		487,604
Stockholders' equity	82,794		82,794		80,331
Total liabilities and stockholders' equity	543,086		543,086		$ 567,935
Income statements:
Net income	$ 1,519	$ 837	$ 4,464	$ 2,122