Investments - Gross Unrealized Gains and Losses and Cost of Equity (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Equity securities, Cost	$ 20,769	$ 19,962
Equity securities, Gross Unrealized Gains	846	30
Equity securities, Gross Unrealized Losses		(1,341)
Equity securities, Fair Value	21,615	18,651
Domestic Common Stock [Member]
Investment [Line Items]
Equity securities, Cost	20,769	19,962
Equity securities, Gross Unrealized Gains	846	30
Equity securities, Gross Unrealized Losses		(1,341)
Equity securities, Fair Value	$ 21,615	$ 18,651