CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Fixed income, amortized cost	$ 4,946,278	$ 4,717,211
Shareholders' equity:
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	1,000,000,000	1,000,000,000
Common stock, shares issued (in shares)	370,567,000	359,400,000
Common stock, shares outstanding (in shares)	370,567,000	340,663,000
Treasury stock, shares at cost (in shares)	0	18,737,000