Debt and Equity Securities (Investments in Equity Securities) (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt And Equity Securities F V N I [Line Items]
Equity securities, Cost	$ 358,352	$ 413,672
Equity securities, Estimated fair value	353,535	466,516
Preferred stocks
Debt And Equity Securities F V N I [Line Items]
Equity securities, Cost	16,892	19,233
Equity securities, Estimated fair value	14,162	18,990
Common stocks
Debt And Equity Securities F V N I [Line Items]
Equity securities, Cost	341,460	394,439
Equity securities, Estimated fair value	$ 339,373	$ 447,526