Investments - Gross Unrealized Gains and Losses and Cost of Equity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Equity securities, Cost or Amortized Cost	$ 19,962	$ 8,503
Equity securities, Gross Unrealized Gains	30	779
Equity securities, Gross Unrealized Losses	(1,341)
Equity securities, Fair Value	18,651	$ 9,282
Domestic Common Stock [Member]
Investment [Line Items]
Equity securities, Cost or Amortized Cost	19,962
Equity securities, Gross Unrealized Gains	30
Equity securities, Gross Unrealized Losses	(1,341)
Equity securities, Fair Value	$ 18,651