Financial Instruments (Details 6) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value of long-term debt		$ 682,900	$ 599,100
Carrying value of long-term debt		673,410	$ 593,659
Unit adjustment liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Method Investment, Realized Gain (Loss) on Disposal		(65,461)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances		(19,145)
Balance at beginning of year		79,934
Change in fair value to the unit adjustment liability		4,672
Contingent Consideration for Acquisition [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at end of year	[1]	$ 0

[1] .