Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Fixed maturities, available-for-sale, amortized cost	$ 3,027,408	$ 2,628,225
Equity securities, available-for-sale, cost	224,159	327,911
Short-term investments, amortized cost	$ 127,125	$ 143,451
Preferred Stock, par value	$ 0.10	$ 0.10
Preferred Stock, authorized shares	1,000,000	1,000,000
Preferred Stock, issued shares	0	0
Common Stock, par value	$ 0.10	$ 0.10
Common Stock, authorized shares	50,000,000	50,000,000
Common Stock, issued shares	36,530,000	36,147,000
Treasury Stock, shares	7,023,000	7,023,000