Consolidated Balance Sheets (Parenthetical) $ in Thousands	Mar. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares
Statement Of Financial Position [Abstract]
Fixed maturities, available-for-sale, amortized cost | $	$ 2,678,475	$ 2,628,225
Equity securities, available-for-sale, cost | $	326,910	327,911
Short-term investments, amortized cost | $	$ 123,796	$ 143,451
Preferred stock, par value | $ / shares	$ 0.10	$ 0.10
Preferred stock, authorized shares	1,000,000	1,000,000
Preferred stock, issued shares	0	0
Common stock, par value | $ / shares	$ 0.10	$ 0.10
Common stock, authorized shares	50,000,000	50,000,000
Common stock, issued shares	36,484,000	36,147,000
Treasury stock, shares	7,023,000	7,023,000