Consolidated Statements of Financial Position (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Fair value of bonds held-to-maturity	$ 8,130,084	$ 7,774,353
Amortized cost of bonds available-for-sale	6,261,621	5,957,901
Cost of equity securities	714,504	757,583
Accumulated depreciation of investment real estate	267,920	260,904
Property and equipment, net of accumulated depreciation	$ 236,922	$ 217,076
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)	26,885,449	26,931,884