Segment Information (Entity-Wide Disclosures) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Interest Expense	$ 5,608	$ 4,351	$ 16,336	$ 11,658
Customer revenues	204,919	254,101	611,566	745,229
Customer Revenues [Member]
Segment Reporting Information [Line Items]
Customer revenues	204,919	254,101	611,566	745,229
CAM [Member]
Segment Reporting Information [Line Items]
Interest Expense	4,700	4,000	13,700	10,900
Customer revenues	58,786	58,465	165,539	155,973
CAM [Member] | Customer Revenues [Member]
Segment Reporting Information [Line Items]
Customer revenues	41,366	40,940	115,270	104,102
All other [Member]
Segment Reporting Information [Line Items]
Customer revenues	22,598	54,542	61,611	153,582
All other [Member] | Customer Revenues [Member]
Segment Reporting Information [Line Items]
Customer revenues	20,117	52,307	53,888	134,249
Ground Services [Member] | All other [Member] | Customer Revenues [Member]
Segment Reporting Information [Line Items]
Customer revenues	$ 19,540	$ 51,728	$ 51,854	$ 132,678