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Derivative Financial Instruments - Changes in Fair Value of Derivative Contracts (Details) - 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, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax:
Other comprehensive gain (loss)	$ 1,086	$ (923)	$ 3,944	$ (304)
Derivative contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax:
Beginning balance			5,853	$ 6,937	$ 6,937
Loss recognized in other comprehensive loss			(3,622)		669
Amounts reclassified from other comprehensive loss to earnings			(297)		(1,563)
Other comprehensive gain (loss)			(3,919)		(894)
Ending balance	$ 1,771		1,771		5,853
Bank loan swap agreements
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax:
Gain recognized in earnings			$ (163)		$ (190)