Financial Instruments (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Interest rate cap hedges recognized in other comprehensive income, net of tax	$ (242,000)	$ (305,000)	$ 0
Other Current Assets
Derivative [Line Items]
Derivative asset	0	0
Other Long Term Assets
Derivative [Line Items]
Derivative asset	165,000	553,000
Interest rate cap premiums
Derivative [Line Items]
Interest rate cap hedges recognized in other comprehensive income, net of tax	200,000
Gain (loss) on derivative, net	0
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate cap premiums | Other Current Assets
Derivative [Line Items]
Derivative asset	0	0
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate cap premiums | Other Long Term Assets
Derivative [Line Items]
Derivative asset	$ 165,000	$ 553,000