Investments - Gross Unrealized Gains and Losses and Cost of Equity (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities, Cost	$ 37,682	$ 35,787
Equity securities, Gross Unrealized Gains	13,161	7,650
Equity securities, Fair Value	50,843	43,437
Domestic Common Stock [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities, Cost	37,682	35,787
Equity securities, Gross Unrealized Gains	13,161	7,650
Equity securities, Fair Value	$ 50,843	$ 43,437