Consolidated Statements of Financial Position (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Fair value of bonds held-to-maturity	$ 9,063,688	$ 8,130,084
Amortized cost of bonds available-for-sale	6,545,358	6,261,621
Cost of equity securities	700,727	714,504
Accumulated depreciation of investment real estate	250,302	267,920
Property and equipment, net of accumulated depreciation	$ 251,832	$ 236,922
Common stock, par value (in usd per share)	$ 1.00	$ 1.00
Common stock, shares authorized (in shares)	50,000,000	50,000,000
Common stock, shares issued (in shares)	30,832,449	30,832,449
Common stock, shares outstanding (in shares)	26,887,200	26,885,449