Condensed Balance Sheets (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Condensed Balance Sheet Statements Captions [Line Items]
Equity securities, cost	$ 1,625,256	$ 2,904,496
Debt securities, amortized cost	13,798,576	11,895,850
Parent Company
Condensed Balance Sheet Statements Captions [Line Items]
Equity securities, cost	1,708	47,546
Debt securities, amortized cost	$ 179,747	$ 90