Significant Acquisitions and Dispositions Acquisition related activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Business Acquisition, Transaction Costs	$ 0	$ (96,000)	$ (499,000)
Business Combination, Step Acquisition, Equity Interest in Acquiree, Remeasurement Gain	0	7,272,000	0
Business Combination, Contingent Consideration, Asset	0	0	(8,000,000)
Acquisition-related activity	$ 0	$ 7,176,000	$ (8,499,000)