Condensed Balance Sheets (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Equity securities, cost	$ 2,816,572	$ 2,740,984
Debt securities, amortized cost	12,927,103	13,529,923
Parent Company
Condensed Financial Statements, Captions [Line Items]
Equity securities, cost	171,860	217,039
Debt securities, amortized cost	$ 31,505	$ 31,746