Debt and Equity Securities (Investments in Equity Securities) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt And Equity Securities F V N I [Line Items]
Equity securities, Cost	$ 322,939	$ 358,352
Equity securities, Estimated fair value	344,790	353,535
Preferred stocks
Debt And Equity Securities F V N I [Line Items]
Equity securities, Cost	17,082	16,892
Equity securities, Estimated fair value	13,888	14,162
Common stocks
Debt And Equity Securities F V N I [Line Items]
Equity securities, Cost	305,857	341,460
Equity securities, Estimated fair value	$ 330,902	$ 339,373