Investments - Gross Unrealized Gains and Losses and Cost of Equity (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investment [Line Items]
Equity securities, Cost	$ 22,582	$ 19,962
Equity securities, Gross Unrealized Gains	1,488	30
Equity securities, Gross Unrealized Losses		(1,341)
Equity securities, Fair Value	24,070	18,651
Domestic Common Stock [Member]
Investment [Line Items]
Equity securities, Cost	22,582	19,962
Equity securities, Gross Unrealized Gains	1,488	30
Equity securities, Gross Unrealized Losses		(1,341)
Equity securities, Fair Value	$ 24,070	$ 18,651