Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Amortized cost on fixed maturities trading investments	$ 2,951,571	$ 2,823,230
Cost - equity security trading investments	619,444	474,197
Cost - short-term investments	263,037	302,693
Fixed assets, accumulated depreciation	$ 354,967	$ 340,523
Common Stock
Common stock, no par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	70,000,000	70,000,000
Common stock, shares issued (in shares)	55,340,000	55,332,000
Common stock, shares outstanding (in shares)	55,340,000	55,332,000