Investments - Summary of Financial Information (Detail) - Donegal Financial Services Corporation [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Balance sheets:
Total assets	$ 559,961		$ 559,961		$ 535,590
Total liabilities	478,455		478,455		457,101
Stockholders' equity	81,506		81,506		78,489
Total liabilities and stockholders' equity	559,961		559,961		$ 535,590
Income statements:
Net income	$ 837	$ 742	$ 2,122	$ 1,449