Investments - Summary of Cost and Estimated Fair Values of Equity Securities (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Equity securities, cost	$ 535,112,451	$ 538,414,337
Equity securities, Gross Gains	2,139,728	5,417,334
Equity securities, Gross Losses	10,693,875	4,885,621
Equity securities, estimated fair value	526,558,304	538,946,050
Equity Securities [Member]
Schedule of Equity Method Investments [Line Items]
Equity securities, cost	40,942,716	44,219,097
Equity securities, Gross Gains	4,817,917	6,505,287
Equity securities, Gross Losses	2,093,624	279,141
Equity securities, estimated fair value	$ 43,667,009	$ 50,445,243