Debt and Equity Securities (Investments in Equity Securities) (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt And Equity Securities F V N I [Line Items]
Equity securities, Cost	$ 369,059	$ 358,352
Equity securities, Estimated fair value	399,048	353,535
Preferred stocks
Debt And Equity Securities F V N I [Line Items]
Equity securities, Cost	17,464	16,892
Equity securities, Estimated fair value	13,440	14,162
Common stocks
Debt And Equity Securities F V N I [Line Items]
Equity securities, Cost	351,595	341,460
Equity securities, Estimated fair value	$ 385,608	$ 339,373