CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Fixed maturities, available for sale, amortized cost (in usd)	$ 7,302,950	$ 7,152,127
Equity securities, available for sale, cost (in usd)	$ 116,320	$ 134,013
Preferred stock, par value (in usd per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized	1,000,000	1,000,000
Preferred stock, shares issued	0	0
Common stock, par value (in usd per share)	$ 0.001	$ 0.001	$ 0.001
Common stock, shares authorized	75,000,000	75,000,000
Common stock, shares issued	65,439,245	64,917,683
Treasury stock, shares	24,721,000	24,672,932	23,971,522	23,308,430