Investments (Narrative) (Details) 1 - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2016	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2008	Aug. 31, 2007
Unrealized gain (loss) on investment under fair value method			$ 14,000	$ 24,800
Gain (Loss) on Disposition of Assets			$ (109)	307
kaleo
Total cash invested in private company								$ 7,500
Ownership interest percentage			20.00%
Carrying value	$ 68,000		$ 68,000			$ 62,200	$ 54,000
Unrealized gain (loss) on investment under fair value method	5,800	$ 21,500	14,000	24,800
Unrealized Gain Loss On Investment Under Fair Value Method After Taxes	$ 4,500	$ 15,700	$ 10,900	$ 18,200
Weighted average cost of capital
Basis point decrease of weighted average cost of capital assumption
Basis point increase of weighted average cost of capital assumption
Alleghany and Bath County, Virginia [Member]
Cost Method Investments	$ 1,600		$ 1,600				1,600
Harbinger Capital Partners Special Situations Fund L P [Member]
Cost-method investment, OTTI	100		100
Cost Method Investments	$ 1,600		$ 1,600				$ 1,700