Investments - Summary of Financial Information (Detail) - Donegal Financial Services Corporation [Member] - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Balance sheets:
Total assets	$ 567,935,408	$ 535,590,133
Total liabilities	487,603,999	457,101,287
Stockholders' equity	80,331,409	78,488,846
Total liabilities and stockholders' equity	567,935,408	535,590,133
Income statements:
Net income	$ 3,362,861	$ 2,252,456	$ 2,372,650