Significant Accounting Policies (Details) - USD ($)	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Option to purchase noncontrolling interest impairment charge		$ 0
Beginning Balance		62,886,000		$ 62,886,000
Redemptions	$ 0		$ (11,947,000)	26,191,000
Granted					$ 11,947,000
Changes in redemption value				(5,965,000)
Ending Balance	55,730,000			55,730,000		$ 62,886,000
Redeemable Noncontrolling Interest
Significant Accounting Policies [Line Items]
Beginning Balance		$ 62,886,000		62,886,000	$ 60,180,000	60,180,000
Redemptions				(8,858,000)		(910,000)
Granted				0		1,666,000
Changes in redemption value				2,062,000		1,498,000
Currency translation adjustments				(360,000)		452,000
Ending Balance	$ 55,730,000			$ 55,730,000		$ 62,886,000