Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Amortized cost on fixed maturities trading investments	$ 3,011,979	$ 2,969,541
Cost - equity security trading investments	577,263	544,082
Cost - short-term investments	367,837	254,518
Fixed assets, accumulated depreciation	$ 364,015	$ 359,269
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)	55,350,000	55,340,000
Common stock, shares outstanding (in shares)	55,350,000	55,340,000