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Investments (Narrative) (Details) 1 - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2008	Aug. 31, 2007
Unrealized gain (loss) on investment under fair value method	$ 8,200	$ 3,300
Gain (Loss) on Disposition of Assets	$ 0	164
kaleo
Total cash invested in private company					$ 7,500
Ownership interest percentage	20.00%
Carrying value	$ 62,200	23,500	$ 20,200	$ 54,000
Unrealized gain (loss) on investment under fair value method	8,200	3,300
Unrealized Gain Loss On Investment Under Fair Value Method After Taxes	$ 6,400	$ 2,500
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
Harbinger Capital Partners Special Situations Fund L P [Member]
Cost Method Investments	$ 1,700			$ 1,700

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