Summary of Investments (Unrealized Appreciation) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Change in net unrealized investment appreciation
Available-for-sale fixed maturities	$ (80,023)	$ 34,837
Available-for-sale equity securities	0	27,559
Deferred policy acquisition costs	7,274	(3,582)
Income tax effect	15,279	(20,584)
Net unrealized investment depreciation of discontinued operations, sold	6,714	0
Cumulative change in accounting principles			$ (191,244)	$ 0
Total change in net unrealized investment appreciation, net of tax	$ (242,000)	$ 38,230